Clip iframe overflow controls.

      Clips overflow controls via m_overflowControlsHostLayer.

BUG=406593

Review URL: https://codereview.chromium.org/564983003

git-svn-id: svn://svn.chromium.org/blink/trunk@182021 bbb929c8-8fbe-4397-9dbb-9b2b20218538