Implement top controls show/hide functionality for main thread scrolling

public/web/WebWidget.h

Index: public/web/WebWidget.h
diff --git a/public/web/WebWidget.h b/public/web/WebWidget.h
index 33cceb12b83554f47f34fb06db9758930bfc4ec4..47db18352cd84d5452b58c35c64caedc7bf2d82d 100644
--- a/public/web/WebWidget.h
+++ b/public/web/WebWidget.h
@@ -37,6 +37,7 @@
 #include "../platform/WebPoint.h"
 #include "../platform/WebRect.h"
 #include "../platform/WebSize.h"
+#include "../platform/WebTopControlsState.h"
 #include "WebBeginFrameArgs.h"
 #include "WebCompositionUnderline.h"
 #include "WebTextDirection.h"
@@ -275,6 +276,10 @@ public:
     // the embedder shrunk the WebView size by the top controls height.
     virtual void setTopControlsHeight(float height, bool topControlsShrinkLayoutSize) { }
 
+    // Updates top controls constraints and current state. Allows embedder to
+    // control what are valid states for top controls and if it should animate.
+    virtual void updateTopControlsState(WebTopControlsState constraints, WebTopControlsState current, bool animate) { }
+
 protected:
     ~WebWidget() { }
 };