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Some tests are still flaky. Use content::RunAllTasksUntilIdle()
instead of RunLoop().RunUntilIdle() to try to run all scheduled
mojo callbacks.

Bug: 1115172
Change-Id: I0146698b9092b2aac48a357b69f4d2d7818d367c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2374187Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801283}