[scheduling] Use ScopedTaskEnvironment instead of MessageLoop in favicon

Use ScopedTaskEnvironment instead of MessageLoop in
favicon_service_impl_unittest.cc

MessageLoop will go away, eventually.

BUG=891670

Change-Id: I05f9b446058cdfe5e649e821fc8d87ac3e6ec764
Reviewed-on: https://chromium-review.googlesource.com/c/1352365
Reviewed-by: Peter Kotwicz <pkotwicz@chromium.org>
Commit-Queue: Carlos Caballero <carlscab@google.com>
Cr-Commit-Position: refs/heads/master@{#612441}
1 file changed