In some cases ANGLE flushes state for FBOs during validation. For
testing of the state synching code for FBOs, this makes end-to-end
testing impossible.

Solve this by partially implementing a hidden no_error extension,
hidden to the user by not exposing the extension string, but allowing
us to skip validation of some of the FBO methods that require checking
for complete FBOs.

BUG=angleproject:1280
BUG=angleproject:1260

Change-Id: I708f348ccec6697b974c48cd890ec75a703abe21
Reviewed-on: https://chromium-review.googlesource.com/322210
Tryjob-Request: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>