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).

include/capi/cef_base_capi.h

@@ -44,62 +44,54 @@ extern "C" {
 #endif
 
 ///
-// All ref-counted framework structures must include this structure first.
+/// All ref-counted framework structures must include this structure first.
 ///
 typedef struct _cef_base_ref_counted_t {
   ///
-  // Size of the data structure.
+  /// Size of the data structure.
   ///
   size_t size;
 
   ///
-  // Called to increment the reference count for the object. Should be called
-  // for every new copy of a pointer to a given object.
+  /// Called to increment the reference count for the object. Should be called
+  /// for every new copy of a pointer to a given object.
   ///
   void(CEF_CALLBACK* add_ref)(struct _cef_base_ref_counted_t* self);
 
   ///
-  // Called to decrement the reference count for the object. If the reference
-  // count falls to 0 the object should self-delete. Returns true (1) if the
-  // resulting reference count is 0.
+  /// Called to decrement the reference count for the object. If the reference
+  /// count falls to 0 the object should self-delete. Returns true (1) if the
+  /// resulting reference count is 0.
   ///
   int(CEF_CALLBACK* release)(struct _cef_base_ref_counted_t* self);
 
   ///
-  // Returns true (1) if the current reference count is 1.
+  /// Returns true (1) if the current reference count is 1.
   ///
   int(CEF_CALLBACK* has_one_ref)(struct _cef_base_ref_counted_t* self);
 
   ///
-  // Returns true (1) if the current reference count is at least 1.
+  /// Returns true (1) if the current reference count is at least 1.
   ///
   int(CEF_CALLBACK* has_at_least_one_ref)(struct _cef_base_ref_counted_t* self);
 } cef_base_ref_counted_t;
 
 ///
-// All scoped framework structures must include this structure first.
+/// All scoped framework structures must include this structure first.
 ///
 typedef struct _cef_base_scoped_t {
   ///
-  // Size of the data structure.
+  /// Size of the data structure.
   ///
   size_t size;
 
   ///
-  // Called to delete this object. May be NULL if the object is not owned.
+  /// Called to delete this object. May be NULL if the object is not owned.
   ///
   void(CEF_CALLBACK* del)(struct _cef_base_scoped_t* self);
 
 } cef_base_scoped_t;
 
-// Check that the structure |s|, which is defined with a size_t member at the
-// top, is large enough to contain the specified member |f|.
-#define CEF_MEMBER_EXISTS(s, f) \
-  ((intptr_t) &                 \
-   ((s)->f) - (intptr_t)(s) + sizeof((s)->f) <= *reinterpret_cast<size_t*>(s))
-
-#define CEF_MEMBER_MISSING(s, f) (!CEF_MEMBER_EXISTS(s, f) || !((s)->f))
-
 #ifdef __cplusplus
 }
 #endif