GitLab

* Fixes issue with an on-receive callback not being unbound, causing
  the incoming message buffer to be drained immediately.
* Add browser tests to verify buffering behavior.
* Add browser tests to verify proper origin enforcement for
  Frame::PostMessage.
* Make the AsyncValueReceiver interface a bit nicer to use.
* Minor cleanups for other FrameImpl browser tests.

Bug: 893236
Change-Id: I521e9316ccf9195133a940be670556a882cb1044
Reviewed-on: https://chromium-review.googlesource.com/c/1351777
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612812}