Rename positionFromPaintInvalidationContainer to positionFromPaintInvalidationBacking

With layer squashing, paintInvalidationContainer is not always the
backing. The position is not always from paintInvalidationContainer,
but from paint invalidation backing.

Review URL: https://codereview.chromium.org/564523002

git-svn-id: svn://svn.chromium.org/blink/trunk@182015 bbb929c8-8fbe-4397-9dbb-9b2b20218538