Finish ReadBuffer support in end2end_tests. Currently we had a few TODOs concerned about unimplemented calls in ANGLE. ANGLE now has support for ReadBuffer, so uncomment the calls. BUG=angleproject:940 Change-Id: Ib297e3ab7f2fa2424ee2cc85918331066dae4475 Reviewed-on: https://chromium-review.googlesource.com/317340 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>