Shader source strings are allowed invalid ESSL characters when they are in
comments.  Added a simple comment parser to determine which characters
should be validated.

BUG=angleproject:2093

Change-Id: If78a4ecbd61f1700fc18dcb844f3de03314a6a39
Reviewed-on: https://chromium-review.googlesource.com/578567
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>