Turn on -Werror on all non-Win platforms.

Part of the patch by mithro@mithis.com, that part reviewed at
https://codereview.chromium.org/47023014

BUG=312287
R=dglazkov@chromium.org
TBR=dglazkov
NOTRY=true

Review URL: https://codereview.chromium.org/50253002

git-svn-id: svn://svn.chromium.org/blink/trunk@161025 bbb929c8-8fbe-4397-9dbb-9b2b20218538