Revert of cc: Re-enable prepaint for GPU rasterization (patchset #1 id:1 of https...

Revert of cc: Re-enable prepaint for GPU rasterization (patchset #1 id:1 of https://codereview.chromium.org/837533002/)

Reason for revert:
Better to leave this in, and now we know that this doesn't improve telemetry significantly

Original issue's description:
> cc: Re-enable prepaint for GPU rasterization
>
> The synchronous rasterization change for GPU rasterization tries
> to avoid prepainting, which may have caused longer frame times in
> some of the tests.
>
> We'll re-enable prepainting to see if this is the reason.  This
> will probably be reverted.
>
> Committed: https://crrev.com/dee056c614737f8bf07686d38dfcc4d264c51813
> Cr-Commit-Position: refs/heads/master@{#309956}

TBR=vmpstr@chromium.org,vmiura@chromium.org
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/844503004

Cr-Commit-Position: refs/heads/master@{#310407}