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/gn_args.py

@@ -87,9 +87,12 @@ else:
   print('Unknown operating system platform')
   sys.exit()
 
+_QUIET = False
+
 
 def msg(msg):
-  print('NOTE: ' + msg)
+  if not _QUIET:
+    print('NOTE: ' + msg)
 
 
 def NameValueListToDict(name_value_list):
@@ -591,12 +594,16 @@ def LinuxSysrootExists(cpu):
   return os.path.isdir(os.path.join(sysroot_root, sysroot_name))
 
 
-def GetAllPlatformConfigs(build_args):
+def GetAllPlatformConfigs(build_args, quiet=False):
   """
   Return a map of directory name to GN args for the current platform.
   """
   result = {}
 
+  if quiet:
+    global _QUIET
+    _QUIET = True
+
   # Merged args without validation.
   args = GetMergedArgs(build_args)