When drawing WebVR, vr_shell_gl.cc's DrawFrame() has some early-return cases to avoid doing VR-shell-related stuff.

Now that the app button toggles menu mode, we'll be popping back and forth between "drawing WebVR" and not.

If a UI element has the controller hovering on it when we exit the menu, we should clean up this state by sending MouseLeave.  More generally, we should run the input state machine whether or not we're drawing WebVR, and let that state machine clean up when we switch modes.

See CL 2768133003.