Refactor FrameToken queue into own class

RenderWidgetHostImpl makes use of an internal queue holding IPC::Messages which
it synchronizes to FrameTokens.

However this will be needed for RenderFrameMetadata observing, to also
synchronize on frame submission. So this refactors it into its own class to be
used by both

[email protected]

Bug: 785013
Change-Id: Icc392444341ed1edbe2994c05054ae305f91f884
Reviewed-on: https://chromium-review.googlesource.com/911829
Reviewed-by: Jonathan Ross <[email protected]>
Reviewed-by: Antoine Labour <[email protected]>
Reviewed-by: Fady Samuel <[email protected]>
Commit-Queue: Jonathan Ross <[email protected]>
Cr-Commit-Position: refs/heads/master@{#538707}
6 files changed