Trace DocumentTimeline::serviceAnimations()

Logging the CPU time of this function will be useful for checking the
overall performance of interpolating AnimatableValue types.

BUG=257591

Review URL: https://codereview.chromium.org/25694002

git-svn-id: svn://svn.chromium.org/blink/trunk@158805 bbb929c8-8fbe-4397-9dbb-9b2b20218538