Certain warnings are suppressed in Chromium but not standalone ANGLE.
We should enable those warnings for ANGLE specifically, in a follow-up
CL.

BUG=angleproject:1382

Change-Id: I12491a61b0f349c5937a1e5d9985c8da1d76e0dc
Reviewed-on: https://chromium-review.googlesource.com/358107Reviewed-by: Jamie Madill <jmadill@chromium.org>