Enabling extensions can cause framebuffers and textures to go from
incomplete to complete.

No functional changes from the API surface.

BUG=angleproject:1523
BUG=angleproject:1958

Change-Id: Iff686e1c292f6cf4921d6fffd7eb98bedaba828b
Reviewed-on: https://chromium-review.googlesource.com/468410
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>