Revert 157920 "Fixed position element should not paint into comp..."

The patch is fine, but I left the description for a previous
version of the patch that is terribly misleading. Will re-land
with correct description so that anyone looking at revision
history will be less confused.

> Fixed position element should not paint into composited ancestor if it is composited for some other reason.
> 
> For a composited element that does not need its own backing (such as
> an overflow clip with composited descendant), the element will actually
> paint its own contents into the composited ancestor. For fixed-position
> elements this is incorrect, since repaint invalidations will not be
> sent.
> 
> This patch forces such fixed-position layers to have their own backing.
> 
> BUG=151734
> 
> Review URL: https://chromiumcodereview.appspot.com/23468020

TBR=shawnsingh@chromium.org

Review URL: https://codereview.chromium.org/25021005

git-svn-id: svn://svn.chromium.org/blink/trunk@158540 bbb929c8-8fbe-4397-9dbb-9b2b20218538