Polymer switchover

As discussed with Sriram, this CL fixes the default UI to be the Polymer one. It also hides the "Switch UI" context menu item by default.

The default UI can still be temporarily enforced locally by modifying the value of DEFAULT_UI_ in background.js.

Also, the "Switch UI" context menu item can be temporarily brought back by modifying the value of CAN_SWITCH_UIS_ in background.js.

@devoncarew (since you've been in this part of the code before
@dinhviethoa @gaurave @keertip (to make you all explicitly aware of the change, and solicit possible objections)