Currently we had a few TODOs concerned about unimplemented calls in
ANGLE. ANGLE now has support for ReadBuffer, so uncomment the calls.

Re-land with end2end_tests fix.

BUG=angleproject:940

Change-Id: I6e00c3aac6307308fb7f00c7e0426812563ed778
Reviewed-on: https://chromium-review.googlesource.com/317821
Tryjob-Request: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>