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

git-svn-id: svn://svn.chromium.org/blink/trunk@157920 bbb929c8-8fbe-4397-9dbb-9b2b20218538