Make YCM ignore unknown warning options.

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)]
 
+  # Version of Clang used to compile Chromium can be newer then version of
+  # libclang that YCM uses for completion. So it's possible that YCM's libclang
+  # doesn't know about some used warning options, which causes compilation
+  # warnings (and errors, because of '-Werror');
+  chrome_flags.append('-Wno-unknown-warning-option')
+
   # 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):
@@ skipping 76 matching lines @@
   """
   chrome_root = FindChromeSrcFromFilename(filename)
   chrome_flags = GetClangCommandFromNinjaForFilename(chrome_root,
                                                      filename)
   final_flags = flags + chrome_flags
 
   return {
     'flags': final_flags,
     'do_cache': True
   }