cc: Attempt to deflake the LayerTreeHostPictureTestTwinLayer test.

cc: Attempt to deflake the LayerTreeHostPictureTestTwinLayer test.

This test has been flaky (see bug), but I am not able to recreate the
flake locally. However, this test does PostSetNeedsCommit() from the
impl thread while also causing commits from the main thread in DidCommit
which could cause an extra commit to happen in theory.

This changes the test to stop using PostSetNeedsCommit() and always
cause commits from DidCommit on the main thread, to help make the test
more deterministic.

R=ajuma, enne@chromium.org, vmpstr
BUG=451988
Committed: https://crrev.com/c949ffe0242ad0459c43a5c9f80f6b29ddb5cbec
Cr-Commit-Position: refs/heads/master@{#313400}

[ajuma, 2015-01-27 23:11:53]

lgtm, the test is much easier to understand now.

[danakj, 2015-01-27 23:20:53]

The CQ bit was checked by danakj@chromium.org

[danakj, 2015-01-27 23:20:58]

The CQ bit was unchecked by danakj@chromium.org

[danakj, 2015-01-27 23:21:10]

The CQ bit was checked by danakj@chromium.org

[commit-bot: I haz the power, 2015-01-27 23:22:30]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/877213002/1

[commit-bot: I haz the power, 2015-01-27 23:36:52]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-27 23:40:11]

Patchset 1 (id:??) landed as
https://crrev.com/c949ffe0242ad0459c43a5c9f80f6b29ddb5cbec
Cr-Commit-Position: refs/heads/master@{#313400}