chromeos: Update FileFlusherTest to not use BrowserThread.

Remove calls to
content::BrowserThread::GetBlockingPool()->FlushForTesting()
and base::RunLoop().RunUntilIdle() from TearDown().
content::TestBrowserThreadBundle is documented as already
running remaining tasks during destruction.

BUG=667892

Change-Id: I0368a0f94e9ddbfde284b18e3222894ee1d80b55
Reviewed-on: https://chromium-review.googlesource.com/560643
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Satoru Takabayashi <satorux@google.com>
Commit-Queue: Dan Erat <derat@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485435}
1 file changed