Improve YouCompleteMe handling of Blink header without source files.

tools/vim/chromium.ycm_extra_conf.py

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Autocompletion config for YouCompleteMe in Chromium.
 #
 # USAGE:
 #
 #   1. Install YCM [https://github.com/Valloric/YouCompleteMe]
 #          (Googlers should check out [go/ycm])
@@ skipping 112 matching lines @@
   Returns:
     (List of Strings) Command line arguments for clang.
   """
   if not chrome_root:
     return []
 
   # Generally, everyone benefits from including Chromium's src/, because all of
   # Chromium's includes are relative to that.
   chrome_flags = ['-I' + os.path.join(chrome_root)]
 
+  # Default file to get a reasonable approximation of the flags for a Blink
+  # file.
+  blink_root = os.path.join(chrome_root, 'third_party', 'WebKit')
+  default_blink_file = os.path.join(blink_root, 'Source', 'core', 'Init.cpp')
+
   # Header files can't be built. Instead, try to match a header file to its
   # corresponding source file.
   if filename.endswith('.h'):
+    # Add config.h to Blink headers, which won't have it by default.
+    if filename.startswith(blink_root):
+      chrome_flags.append('-include')
+      chrome_flags.append(os.path.join(blink_root, 'Source', 'config.h'))
+
     alternates = ['.cc', '.cpp']
     for alt_extension in alternates:
       alt_name = filename[:-2] + alt_extension
       if os.path.exists(alt_name):
         filename = alt_name
         break
     else:
-      # If this is a standalone .h file with no source, the best we can do is
-      # try to use the default flags.
-      return chrome_flags
+      if filename.startswith(blink_root):
+        # If this is a Blink file, we can at least try to get a reasonable
+        # approximation.
+        filename = default_blink_file
+      else:
+        # If this is a standalone .h file with no source, the best we can do is
+        # try to use the default flags.
+        return chrome_flags
 
   # Ninja needs the path to the source file from the output build directory.
   # Cut off the common part and /.
   subdir_filename = filename[len(chrome_root)+1:]
   rel_filename = os.path.join('..', '..', subdir_filename)
 
   out_dir = GetNinjaOutputDirectory(chrome_root)
 
   # Ask ninja how it would build our source file.
   p = subprocess.Popen(['ninja', '-v', '-C', out_dir, '-t',
@@ skipping 48 matching lines @@
   """
   chrome_root = FindChromeSrcFromFilename(filename)
   chrome_flags = GetClangCommandFromNinjaForFilename(chrome_root,
                                                      filename)
   final_flags = flags + chrome_flags
 
   return {
     'flags': final_flags,
     'do_cache': True
   }