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
Commit-Queue: Antoine Labour <>
Commit-Queue: James Cook <>
Reviewed-by: James Cook <>
Reviewed-by: Yuri Wiitala <>
Auto-Submit: Antoine Labour <>
Cr-Commit-Position: refs/heads/master@{#642669}
5 files changed