[cleanup] Some cleanup in cc::DelayedUniqueNotifier

. Zero delay is not used anywhere. So remove support for zero delay.
. The notifier is only used in the compositor thread, so there is no
  need to use base::Lock. So this is removed, and replaced by a thread
  checker instead.

BUG=none

Change-Id: I1c998a011b9b28edab6b46ac9d772969bb4bb147
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2391981


Commit-Queue: Khushal <khushalsagar@chromium.org>
Auto-Submit: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Khushal <khushalsagar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805669}