Enable warnings for clang on non-mac

I did local debug and release builds on linux,
this works now.

BUG=312287
TBR=dglazkov

Review URL: https://codereview.chromium.org/50363007

git-svn-id: svn://svn.chromium.org/blink/trunk@161088 bbb929c8-8fbe-4397-9dbb-9b2b20218538