Simplify SyncToken logic in CanvasResource

CanvasResource was deferring some calls to WaitSyncToken.
There is no evidence that this deferral affect performance and
it adds unnecessary complexity to the code that makes refactoring
harder and more bug prone.  Therefore, the logic is being removed.

BUG=788439
NOTRY=true

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I9864d2ccdd300b535ba9292373577bd09f9f5f7c
Reviewed-on: https://chromium-review.googlesource.com/1080048
Commit-Queue: Justin Novosad <junov@chromium.org>
Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Reviewed-by: Justin Novosad <junov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#563345}
4 files changed