Replacing raw pointer to LayerAnimator with scoped_refptr in ScopedLayerAnimationSettings.
The raw pointer can cause issues: if the ScopedLayerAnimationSettings object survives the
respective LayerAnimator object, a seg fault occurs when ScopedLayerAnimationSettings is deleted.
This is particularly problematic when the animation in question has a 0 duration (which can be
the case for unit tests). When animation's duration is 0, the Animator gets deleted right away,
by the same setter that starts the animation.

BUG=

Review URL: https://codereview.chromium.org/185793005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255482 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255482