In WebGL, generate INVALID_OPERATION if constant color and constant
alpha are used together as source and destination blend functions.

BUG=angleproject:1817

Change-Id: I9b2d05ab5017c013bb89c13256efbd80198de91b
Reviewed-on: https://chromium-review.googlesource.com/448940Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>