Disable scrolling for fullscreen elements by changing scroll layer size.

After https://codereview.chromium.org/476113004 CC depends on the
tree structure of scroll/clip layers to determine the viewport size, so
detaching them breaks fullscreen mode.  Switch to the other
approach of changing the scroll layer bounds.

BUG=410452

Review URL: https://codereview.chromium.org/558813003

git-svn-id: svn://svn.chromium.org/blink/trunk@181926 bbb929c8-8fbe-4397-9dbb-9b2b20218538