Made root translation layer the root when using pinch virtual viewport.

The "root" translation layer was previously below the overlap controls
host layer. This meant it was actually below the pinch viewport's scale
layer. We really want the root translation layer to be at the root so that
DevTools' screen emulation feature can scale and offset the entire tree.

BUG=370035

Review URL: https://codereview.chromium.org/308183002

git-svn-id: svn://svn.chromium.org/blink/trunk@175496 bbb929c8-8fbe-4397-9dbb-9b2b20218538