Causes crashes in the dEQP ReadPixels test.

BUG=angleproject:1149
BUG=angleproject:883

Change-Id: I090e21ebd7781ce3372b8842421e7da3e4bedbac
Reviewed-on: https://chromium-review.googlesource.com/287629Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>