Deflake AddFirstWallpaperAnimationEndCallback test

This test is currently flaky because it relies upon the ordering
of delayed and non delayed tasks to quit a run loop before the
very short (0.01s) animation delay is up. This patch deflakes it by
temporarily freezing time to prevent the animation from
completing before the test expects it to.

TBR=wzang@chromium.org

Bug: 863341, 891670, 910493
Change-Id: I34cf2824c387c380134eb38b3375e6ab79de4be8
Reviewed-on: https://chromium-review.googlesource.com/c/1365244
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614322}
1 file changed