Remove Mailbox functions from GLHelper

The only caller is CanvasCaptureHandler::ReadYUVPixelsAsync but it
passes around a texture from the same context, so going through
mailboxes is not useful.

Bug: 870036, 944175
Change-Id: I33ed85366f0e63bc568c0332f8fbc895a6fb320e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1532664
Commit-Queue: Antoine Labour <piman@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Yuri Wiitala <miu@chromium.org>
Auto-Submit: Antoine Labour <piman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642669}
5 files changed