viz: Reduce Usage of GetOverlayValidator from OutputSurface

OverlayCandidateValidator is used to validate overlay and is only used
inside OverlayProcessor. This is the first CL to move the ownership of
the validator to the overlay processor. This reduces the usage of
trying to get the validator from the output surface.

R=rjkroege

Bug: 963597
Change-Id: I3541b8c4210db98862e430d23479eb8cb83698ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1613581


Commit-Queue: weiliangc <weiliangc@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660544}