Make YCM ignore unknown warning options.

tools/vim/chromium.ycm_extra_conf.py

Index: tools/vim/chromium.ycm_extra_conf.py
diff --git a/tools/vim/chromium.ycm_extra_conf.py b/tools/vim/chromium.ycm_extra_conf.py
index 71863115871ef2288cc612705101b9f42af5c61e..f26d4beae4af03e0a1ebb537b912d1acdfd90f92 100644
--- a/tools/vim/chromium.ycm_extra_conf.py
+++ b/tools/vim/chromium.ycm_extra_conf.py
@@ -130,6 +130,12 @@ def GetClangCommandFromNinjaForFilename(chrome_root, filename):
   # 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')