Set root clip layer not to maskToBounds.

The root layer doesn't need to clip because it's drawn to a correctly
sized surface.  The only reason it might be needed is to avoid
overlapping the scrollbars, but that's already dealt with by tree
order.  The root clip causes a lot of bugs in dynamic viewport scenarios
on Android, and it's not needed elsewhere, so disable masksToBounds
on it.  Even though it's now a no-op, we leave it in the tree structure
for consistency and because the clip bounds still contain useful
information.

Also rename "clip layer" to "container layer" since it now doesn't
necessarily clip.

NOTRY=true
BUG=259141

Review URL: https://chromiumcodereview.appspot.com/19019004

git-svn-id: svn://svn.chromium.org/blink/trunk@154172 bbb929c8-8fbe-4397-9dbb-9b2b20218538