cc: Fix flakiness in LayerTreeHostCopyRequestTestMultipleDrawsHiddenCopyRequest

cc: Fix flakiness in LayerTreeHostCopyRequestTestMultipleDrawsHiddenCopyRequest

This test was expecting the main thread copy request to happen before a
new draw happens on the compositor thread which is racey. Instead do
this comparison on the main thread by posting and trying to end the test
after the draw happens but checking for the copy request result there
on the main thread instead of on the impl thread before posting.

R=vmpstr, weiliangc
BUG=439649
Committed: https://crrev.com/c67de28bb2624a02fb1937b0f9c9b8ee160370c9
Cr-Commit-Position: refs/heads/master@{#313797}

[vmpstr, 2015-01-29 21:18:12]

lgtm

[danakj, 2015-01-29 21:20:42]

The CQ bit was checked by danakj@chromium.org

[commit-bot: I haz the power, 2015-01-29 21:21:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/888663003/1

[commit-bot: I haz the power, 2015-01-29 21:39:26]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-29 21:40:23]

Patchset 1 (id:??) landed as
https://crrev.com/c67de28bb2624a02fb1937b0f9c9b8ee160370c9
Cr-Commit-Position: refs/heads/master@{#313797}