Revert 120074 - Disable animations during aura tests.

Revert 120074 - Disable animations during aura tests.

This causes all animations scheduled during a test to complete immediately.

After making this change, I noticed some code assumed that animations would not complete synchronously. Some of this code used animation observers, and I while fixing the code I have updated it to use the preferred ImplicitAnimationObserver.

BUG=None
TEST=aura_shell_unittests,aura_unittests,compositor_unittests

Review URL: https://chromiumcodereview.appspot.com/9222018

TBR=vollick@google.com
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=120075