Move canResize to RenderBox

Now that we have transitioned RenderLayerScrollableArea
to RenderBox, we can move canResize to where it belongs.

Review URL: https://codereview.chromium.org/25583003

git-svn-id: svn://svn.chromium.org/blink/trunk@158661 bbb929c8-8fbe-4397-9dbb-9b2b20218538