[fsp] Fix flaky browser test for aborting a write operation.

[fsp] Fix flaky browser test for aborting a write operation.

Before, we were using a timer in hope that the operation is started
synchronously, but it's a wrong assumption.

This patch resolves this issue by waiting until writing is really
started.

TEST=browser_test: *FileSystemProvider*Write*
BUG=426549
Committed: https://crrev.com/64b7e99ac3f6cb68d64c0de3e23fa018cde58fd7
Cr-Commit-Position: refs/heads/master@{#301051}

[mtomasz, 2014-10-24 03:48:05]

mtomasz@chromium.org changed reviewers:
+ fukino@chromium.org

[mtomasz, 2014-10-24 03:48:20]

@fukino: PTAL. Thanks!

[fukino, 2014-10-24 03:53:05]

On 2014/10/24 03:48:20, mtomasz wrote:
> @fukino: PTAL. Thanks!

lgtm. Thanks!

[mtomasz, 2014-10-24 03:53:38]

The CQ bit was checked by mtomasz@chromium.org

[commit-bot: I haz the power, 2014-10-24 03:54:55]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/677593002/1

[commit-bot: I haz the power, 2014-10-24 04:55:50]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-10-24 04:57:10]

Patchset 1 (id:??) landed as
https://crrev.com/64b7e99ac3f6cb68d64c0de3e23fa018cde58fd7
Cr-Commit-Position: refs/heads/master@{#301051}