Disable threaded pixel-moving filter animations

This makes pixel-moving filter animations (that this, those that involve
blur or drop-shadow) run on the main thread rather than the compositor
thread.

Before we can re-enable running such animations on the compositor thread:
1) We need to update overlap testing to use the animated bounds of layers
   running filter animations.
2) We need the compositor to update layer bounds using animated filter
   outsets as a filter animation progresses.

BUG=181613

Review URL: https://codereview.chromium.org/50903008

git-svn-id: svn://svn.chromium.org/blink/trunk@161164 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove virtual where not needed in HTMLMediaElement/HTMLVideoElement

These functions were neither overrides or overridden anywhere, so
the virtual keywords were likely copypasta or unrealized plans.

BUG=313702

Review URL: https://codereview.chromium.org/53453004

git-svn-id: svn://svn.chromium.org/blink/trunk@161166 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Fix CompositedLayerMapping::startTransition's handling of filters

This removes an invalid assert in CompositedLayerMapping::startTransition,
and makes startTransition respect the return value of
GraphicsLayer::addAnimation when adding a filter animation.

BUG=181613

Review URL: https://codereview.chromium.org/52713009

git-svn-id: svn://svn.chromium.org/blink/trunk@161167 bbb929c8-8fbe-4397-9dbb-9b2b20218538