CSS clip property should not force additional clamping to border box

For a composited layer computing its bounds, there is a code path that
clamps a layer's cliprect to the corresponding element's border box.
However, this is incorrect, since a CSS clip may have valid bounds that
include things outside the border box. So, that clamping should only occur
if there is an overflow clip.

BUG=246728
R=jchaffraix@chromium.org

Review URL: https://codereview.chromium.org/17155024

git-svn-id: svn://svn.chromium.org/blink/trunk@152919 bbb929c8-8fbe-4397-9dbb-9b2b20218538