Implement overscroll support for the virtual keyboard.

content/renderer/render_widget.h

Index: content/renderer/render_widget.h
diff --git a/content/renderer/render_widget.h b/content/renderer/render_widget.h
index 809d3630b5f0ca19324a455597840741030174db..270058201bc10e89c4a2c69617227cb57d9a15da 100644
--- a/content/renderer/render_widget.h
+++ b/content/renderer/render_widget.h
@@ -320,6 +320,7 @@ class CONTENT_EXPORT RenderWidget
   void Resize(const gfx::Size& new_size,
               const gfx::Size& physical_backing_size,
               float overdraw_bottom_height,
+              const gfx::Size& visible_viewport_size,
               const gfx::Rect& resizer_rect,
               bool is_fullscreen,
               ResizeAck resize_ack);
@@ -551,6 +552,9 @@ class CONTENT_EXPORT RenderWidget
   // the browser, for example by an on-screen-keyboard (in DPI-adjusted pixels).
   float overdraw_bottom_height_;
 
+  // The size of the visible viewport in DPI-adjusted pixels.
+  gfx::Size visible_viewport_size_;
+
   // The area that must be reserved for drawing the resize corner.
   gfx::Rect resizer_rect_;