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).

tools/make_version_header.py

@@ -12,9 +12,6 @@ import sys
 
 
 def make_version_header(header):
-  if not git.is_checkout('.'):
-    raise Exception('Not a valid checkout')
-
   result = get_copyright(full=True, translator=False) + \
 """//
 // ---------------------------------------------------------------------------
@@ -41,33 +38,6 @@ def make_version_header(header):
 #define DO_MAKE_STRING(p) #p
 #define MAKE_STRING(p) DO_MAKE_STRING(p)
 
-#ifndef APSTUDIO_HIDDEN_SYMBOLS
-
-#include "include/internal/cef_export.h"
-
-#ifdef __cplusplus
-extern "C" {
-#endif
-
-// Returns CEF version information for the libcef library. The |entry|
-// parameter describes which version component will be returned:
-// 0 - CEF_VERSION_MAJOR
-// 1 - CEF_VERSION_MINOR
-// 2 - CEF_VERSION_PATCH
-// 3 - CEF_COMMIT_NUMBER
-// 4 - CHROME_VERSION_MAJOR
-// 5 - CHROME_VERSION_MINOR
-// 6 - CHROME_VERSION_BUILD
-// 7 - CHROME_VERSION_PATCH
-///
-CEF_EXPORT int cef_version_info(int entry);
-
-#ifdef __cplusplus
-}
-#endif
-
-#endif  // APSTUDIO_HIDDEN_SYMBOLS
-
 #endif  // CEF_INCLUDE_CEF_VERSION_H_
 """