Add initial support for API versioning (see #3836)

- Generated files are now created when running cef_create_projects or
  the new version_manager.py tool. These files are still created in the
  cef/ source tree (same location as before) but Git ignores them due to
  the generated .gitignore file.
- API hashes are committed to Git as a new cef_api_versions.json file.
  This file is used for both code generation and CEF version calculation
  (replacing the previous usage of cef_api_hash.h for this purpose).
  It will be updated by the CEF admin before merging breaking API
  changes upstream.
- As an added benefit to the above, contributor PRs will no longer
  contain generated code that is susceptible to frequent merge conflicts.
- From a code generation perspective, the main difference is that we now
  use versioned structs (e.g. cef_browser_0_t instead of cef_browser_t)
  on the libcef (dll/framework) side. Most of the make_*.py tool changes
  are related to supporting this.
- From the client perspective, you can now define CEF_API_VERSION in the
  project configuration (or get CEF_EXPERIMENTAL by default). This
  define will change the API exposed in CEF’s include/ and include/capi
  header files. All client-side targets including libcef_dll_wrapper
  will need be recompiled when changing this define.
- Examples of the new API-related define usage are provided in
  cef_api_version_test.h, api_version_test_impl.cc and
  api_version_unittest.cc.

To test:
- Run `ceftests --gtest_filter=ApiVersionTest.*`
- Add `cef_api_version=13300` to GN_DEFINES. Re-run configure, build and
  ceftests steps.
- Repeat with 13301, 13302, 13303 (all supported test versions).

libcef_dll/cpptoc/cpptoc_ref_counted.h

@@ -45,7 +45,8 @@ class CefCppToCRefCounted : public CefBaseRefCounted {
     }
 
     // Cast our structure to the wrapper structure type.
-    WrapperStruct* wrapperStruct = GetWrapperStruct(s);
+    WrapperStruct* wrapperStruct =
+        GetWrapperStruct(s, /*require_exact_type=*/false);
 
     // If the type does not match this object then we need to unwrap as the
     // derived type.
@@ -68,8 +69,6 @@ class CefCppToCRefCounted : public CefBaseRefCounted {
   static CefRefPtr<BaseName> Get(StructName* s) {
     DCHECK(s);
     WrapperStruct* wrapperStruct = GetWrapperStruct(s);
-    // Verify that the wrapper offset was calculated correctly.
-    DCHECK_EQ(kWrapperType, wrapperStruct->type_);
     return wrapperStruct->object_;
   }
 
@@ -122,13 +121,21 @@ class CefCppToCRefCounted : public CefBaseRefCounted {
     StructName struct_;
   };
 
-  static WrapperStruct* GetWrapperStruct(StructName* s) {
+  static WrapperStruct* GetWrapperStruct(StructName* s,
+                                         bool require_exact_type = true) {
     // Offset using the WrapperStruct size instead of individual member sizes
     // to avoid problems due to platform/compiler differences in structure
     // padding.
-    return reinterpret_cast<WrapperStruct*>(
+    auto* wrapperStruct = reinterpret_cast<WrapperStruct*>(
         reinterpret_cast<char*>(s) -
         (sizeof(WrapperStruct) - sizeof(StructName)));
+
+    if (require_exact_type) {
+      // Verify that the wrapper offset was calculated correctly.
+      CHECK_EQ(kWrapperType, wrapperStruct->type_);
+    }
+
+    return wrapperStruct;
   }
 
   // Unwrap as the derived type.
@@ -152,9 +159,6 @@ class CefCppToCRefCounted : public CefBaseRefCounted {
 
     WrapperStruct* wrapperStruct =
         GetWrapperStruct(reinterpret_cast<StructName*>(base));
-    // Verify that the wrapper offset was calculated correctly.
-    DCHECK_EQ(kWrapperType, wrapperStruct->type_);
-
     wrapperStruct->wrapper_->AddRef();
   }
 
@@ -166,9 +170,6 @@ class CefCppToCRefCounted : public CefBaseRefCounted {
 
     WrapperStruct* wrapperStruct =
         GetWrapperStruct(reinterpret_cast<StructName*>(base));
-    // Verify that the wrapper offset was calculated correctly.
-    DCHECK_EQ(kWrapperType, wrapperStruct->type_);
-
     return wrapperStruct->wrapper_->Release();
   }
 
@@ -180,9 +181,6 @@ class CefCppToCRefCounted : public CefBaseRefCounted {
 
     WrapperStruct* wrapperStruct =
         GetWrapperStruct(reinterpret_cast<StructName*>(base));
-    // Verify that the wrapper offset was calculated correctly.
-    DCHECK_EQ(kWrapperType, wrapperStruct->type_);
-
     return wrapperStruct->wrapper_->HasOneRef();
   }
 
@@ -195,9 +193,6 @@ class CefCppToCRefCounted : public CefBaseRefCounted {
 
     WrapperStruct* wrapperStruct =
         GetWrapperStruct(reinterpret_cast<StructName*>(base));
-    // Verify that the wrapper offset was calculated correctly.
-    DCHECK_EQ(kWrapperType, wrapperStruct->type_);
-
     return wrapperStruct->wrapper_->HasAtLeastOneRef();
   }