Validate that framebuffer attachments all have the same size for WebGL. TEST=conformance2/rendering/draw-buffers BUG=angleproject:1989 Change-Id: Ifcbc766dea99a8361261713fde7a6b6ad724fd55 Reviewed-on: https://chromium-review.googlesource.com/475083 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>