Replace GetBlockingPool()->FlushForTesting() with RunAllBlockingPoolTasksUntilIdle() in prerender unittests.

RunAllBlockingPoolTasksUntilIdle() also flushes the main loop so an
explicit RunEvents() becomes unecessary.

R=davidben@chromium.org

Bug: 667892
Change-Id: Iecf2e386c55cacadb9f064509ed5b109ce70e23a
Reviewed-on: https://chromium-review.googlesource.com/583569
Reviewed-by: David Benjamin <davidben@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#489039}
1 file changed