Remove usage of ScopedAsyncTaskScheduler from TestBrowserThreadBundle.

ScopedAsyncTaskScheduler is deprecated.

Bug: 708584
Change-Id: I795e7d07947369c202ea767b05543bb44474ff47
Reviewed-on: https://chromium-review.googlesource.com/591788
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Francois Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491383}
3 files changed