Don't create overlay scrollbars on main frame in pinch virtual viewport.

In pinch virtual viewport, the inner viewport has scrollbars created
compositor-side to let the user know their position relative to the
top-level document. The main frame scrollbars in Android (and CrOS when
using overlay scrollbars) are redundant with these scrollbars.

This patch short circuits the main frame's updateScrollbars() method
when using pinch virtual viewport and overlay scrollbars. This prevents
the scrollbars from being created or any work done to update them.

BUG=344568

Review URL: https://codereview.chromium.org/542883005

git-svn-id: svn://svn.chromium.org/blink/trunk@182240 bbb929c8-8fbe-4397-9dbb-9b2b20218538