GitLab

Indication of back gesture completion from application comes over an
asynchronous mojo callback and its return value is currently dropped
on the floor in order to avoid blocking the browser process to wait.

This cl changes the ConsumeGesture call to take a completion callback
and along with corresponding internal changes wires the return value
all the way.

Bug: internal b/149740759
Test: manual and unit tests
Change-Id: Ib2e519326e019148a64966e5e46ee9df00d40755
Merge-With: eureka-internal/382879
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2132980Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Commit-Queue: Ryan Daum <rdaum@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756780}