[Mac] Adds a new view that will allow us to animate the height property of views.  This class will be the base class for any view that needs to be animated.

BUG=None
TEST=No visible impact.  New unittests should pass.
Review URL: http://codereview.chromium.org/350002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@30852 0039d316-1c4b-4281-b951-d872f2087c98