Fix flaky animation tests for display none

The failure in
virtual/threaded/animations/display-none-terminates-animation.html looks like
the kind of flakiness we see in lots of animation tests in the
virtual/threaded/ suite. This change moves the failure to an existing bug for
this.

The flakiness in
virtual/web-animations-css/animations/display-change-does-not-terminate-animation.html
appears to be due to a bug in the test. This change makes a fix, and adds to
the output on failure for future debugging.

TBR=eae
BUG=318777,248938

Review URL: https://codereview.chromium.org/63633004

git-svn-id: svn://svn.chromium.org/blink/trunk@161997 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Web Animations: Add hasStarted to DocumentTimeline

This will be used in delaying animations from being dispatched to the
compositor until the corresponding timeline has started.

Review URL: https://codereview.chromium.org/71453006

git-svn-id: svn://svn.chromium.org/blink/trunk@161998 bbb929c8-8fbe-4397-9dbb-9b2b20218538