If a glReadPixels is done after a swapBuffers, it's results are often
undefined.

bug=angleproject:967

Change-Id: I4a99ca47624347ecbc0a5c084e93b997c92e1b31
Reviewed-on: https://chromium-review.googlesource.com/272423Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>