This improves the performance of Framebuffer::hasTextureAttachment 3X.

BUG=angleproject:2188

Change-Id: I6acc7f4440fe0232438182274d7bba6075530f38
Reviewed-on: https://chromium-review.googlesource.com/1159171Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>