ui: Cleanup ui::Layer

* Add & update description of some member functions and variables.
* Remove unused variable - layer_updated_externally_.
* Remove redundant function calls - SetXXXImmediately().
    - Immediate setting is done by default animator.

R=danakj@chromium.org
BUG=NONE
TEST=compositor_unittests

Review URL: https://codereview.chromium.org/140443002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245357 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245357