WebGL validation of constant color & alpha blend. 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/448940 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Frank Henigman <fjhenigman@chromium.org>