Implement overscroll support for the virtual keyboard.

ash/shelf/shelf_layout_manager.cc

Index: ash/shelf/shelf_layout_manager.cc
diff --git a/ash/shelf/shelf_layout_manager.cc b/ash/shelf/shelf_layout_manager.cc
index 2a248050bfa5f3846fcb22ecfabab16f13773c8f..89802cb8d004d70d2a2b37e8ae61e8c4e74e2937 100644
--- a/ash/shelf/shelf_layout_manager.cc
+++ b/ash/shelf/shelf_layout_manager.cc
@@ -46,6 +46,7 @@
 #include "ui/events/event.h"
 #include "ui/events/event_handler.h"
 #include "ui/gfx/screen.h"
+#include "ui/keyboard/keyboard_util.h"
 #include "ui/views/widget/widget.h"
 #include "ui/wm/public/activation_client.h"
 
@@ -846,7 +847,7 @@ void ShelfLayoutManager::CalculateTargetBounds(
   // remove entirely the dependency on keyboard and dock.
 
   // Also push in the work area inset for the keyboard if it is visible.
-  if (!keyboard_bounds_.IsEmpty()) {
+  if (!keyboard_bounds_.IsEmpty() && !keyboard::IsKeyboardOverscrollEnabled()) {
     gfx::Insets keyboard_insets(0, 0, keyboard_bounds_.height(), 0);
     target_bounds->work_area_insets += keyboard_insets;
   }