Implement overscroll support for the virtual keyboard.

ash/root_window_controller.cc

Index: ash/root_window_controller.cc
diff --git a/ash/root_window_controller.cc b/ash/root_window_controller.cc
index 3b1fbce32464198103344831a8c90055172747f7..5a155b46115e2fd26bc6ac894e8d9d947296d1b2 100644
--- a/ash/root_window_controller.cc
+++ b/ash/root_window_controller.cc
@@ -612,6 +612,7 @@ void RootWindowController::ActivateKeyboard(
   }
   DCHECK(keyboard_controller);
   if (!keyboard::IsKeyboardUsabilityExperimentEnabled()) {
+    aura::client::SetVirtualKeyboardClient(root_window(), keyboard_controller);
     keyboard_controller->AddObserver(shelf()->shelf_layout_manager());
     keyboard_controller->AddObserver(panel_layout_manager_);
     keyboard_controller->AddObserver(docked_layout_manager_);
@@ -645,6 +646,7 @@ void RootWindowController::DeactivateKeyboard(
       // Virtual keyboard may be deactivated while still showing, notify all
       // observers that keyboard bounds changed to 0 before remove them.
       keyboard_controller->NotifyKeyboardBoundsChanging(gfx::Rect());
+      aura::client::SetVirtualKeyboardClient(root_window(), NULL);
       keyboard_controller->RemoveObserver(shelf()->shelf_layout_manager());
       keyboard_controller->RemoveObserver(panel_layout_manager_);
       keyboard_controller->RemoveObserver(docked_layout_manager_);