Revert of [Android] Mark posted UI thread tasks as asynchronous (patchset #4 id:60001...

Revert of [Android] Mark posted UI thread tasks as asynchronous (patchset #4 id:60001 of https://codereview.chromium.org/512333002/)

Reason for revert:
This patch was purely experimental, and the experiment has expired.

Original issue's description:
> [Android] Mark posted UI thread tasks as asynchronous
>
> Chromium shares a message loop with Android on the browser UI thread.
> This can cause problems when the associated Looper has a sync barrier,
> preventing posted Chromium tasks from being dispatched until the
> barrier is removed. Make this sharing more fair by marking all Chromium
> Message tasks as asynchronous, avoiding stalls when there is a sync
> barrier.
>
> Note: This change is for gathering data about the perf impact and we'll
> revert it before cutting a release branch.
>
> BUG=407149,380781,407133
>
> Committed: https://crrev.com/feabeebb3ac5810f896ac8303a77ee695acaf9d4
> Cr-Commit-Position: refs/heads/master@{#292551}

TBR=aelias@chromium.org,epenner@chromium.org,nyquist@chromium.org,sievers@chromium.org,skyostil@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=407149,380781,407133

Review URL: https://codereview.chromium.org/564373005

Cr-Commit-Position: refs/heads/master@{#294875}