[fsp] Fix flaky tests for aborting read.

[fsp] Fix flaky tests for aborting read.

If reading is aborted before the file is opened, then obviously abort event is
not dispatched to the providing extension, as there is no operation to abort.

This patch fixes this race by waiting until the read operation is started, so
the abort event will be always dispatched.

TEST=browser_tests: *FileSystemProvider*ReadFile*
BUG=None

Committed: https://crrev.com/8a1436c88f0bc8f6ab62649ed1a9a102f429e9af
Cr-Commit-Position: refs/heads/master@{#310745}

[mtomasz, 2015-01-09 08:25:12]

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

[mtomasz, 2015-01-09 08:25:43]

@yawano: One more small quick fix for a recent CL. PTAL. Thanks.

[yawano, 2015-01-09 08:34:59]

lgtm. Thank you!

[mtomasz, 2015-01-09 08:37:10]

Thanks for fast response!

[mtomasz, 2015-01-09 08:37:13]

The CQ bit was checked by mtomasz@chromium.org

[commit-bot: I haz the power, 2015-01-09 08:37:26]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/836613008/1

[commit-bot: I haz the power, 2015-01-09 09:34:03]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-09 09:35:24]

Patchset 1 (id:??) landed as
https://crrev.com/8a1436c88f0bc8f6ab62649ed1a9a102f429e9af
Cr-Commit-Position: refs/heads/master@{#310745}