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 &quotclip layer&quot to &quotcontainer layer&quot 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