Support API versioning in platform-specific headers (see #3836)

- Exclude platform-specific includes (anything in < >) from the
  clang preprocessor by using `!defined(GENERATING_CEF_API_HASH)`
  in CEF header files.
- Define "target platforms" by passing platform- and architecture-
  specific ifdefs to the clang preprocessor. Grep for `defined(OS_`
  to identify headers that require target platform processing, and
  then process for each target as the platform-specific API hash
  contribution.
- Delete the univeral hash which is no longer a useful concept.

tools/version_util.py

@@ -92,7 +92,7 @@ def read_version_files(api_versions_file,
       }
       initialized = True
     else:
-      json_version = None
+      json_versions = None
   else:
     assert 'hashes' in json_versions, api_versions_file
     assert 'last' in json_versions, api_versions_file