Enforces default compiler options when compiling WebGL compatible shaders.

BUG=angleproject:1616

Change-Id: I18490db68b29981fab4817bdd61727752cf50997
Reviewed-on: https://chromium-review.googlesource.com/409016
Commit-Queue: Bryan Bernhart <bryan.bernhart@intel.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>