Add flag to enable immersive fullscreen for v2 apps

ash/wm/caption_buttons/frame_caption_button_container_view.cc

Index: ash/wm/caption_buttons/frame_caption_button_container_view.cc
diff --git a/ash/wm/caption_buttons/frame_caption_button_container_view.cc b/ash/wm/caption_buttons/frame_caption_button_container_view.cc
index d0b0b96da67577cbcb5ac8752b7cde733b5bca11..41f9e089a8ee35b5117238ab2555dddbc8e660b3 100644
--- a/ash/wm/caption_buttons/frame_caption_button_container_view.cc
+++ b/ash/wm/caption_buttons/frame_caption_button_container_view.cc
@@ -114,6 +114,12 @@ FrameCaptionButtonContainerView::FrameCaptionButtonContainerView(
 FrameCaptionButtonContainerView::~FrameCaptionButtonContainerView() {
 }
 
+FrameMaximizeButton*
+FrameCaptionButtonContainerView::GetOldStyleSizeButton() {
+  return switches::UseAlternateFrameCaptionButtonStyle() ?
+      NULL : static_cast<FrameMaximizeButton*>(size_button_);
+}
+
 void FrameCaptionButtonContainerView::ResetWindowControls() {
   minimize_button_->SetState(views::CustomButton::STATE_NORMAL);
   size_button_->SetState(views::CustomButton::STATE_NORMAL);