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 c37943f096602398d5f354f3e4c700e7350f40ba..db279c36859e6345259a15e0d0740db011881d16 100644
--- a/content/renderer/render_widget.h
+++ b/content/renderer/render_widget.h
@@ -324,6 +324,7 @@ class CONTENT_EXPORT RenderWidget
   void Resize(const gfx::Size& new_size,
               const gfx::Size& physical_backing_size,
               float overdraw_bottom_height,
+              int visible_viewport_height,
               const gfx::Rect& resizer_rect,
               bool is_fullscreen,
               ResizeAck resize_ack);
@@ -567,6 +568,9 @@ class CONTENT_EXPORT RenderWidget
   // the browser, for example by an on-screen-keyboard (in DPI-adjusted pixels).
   float overdraw_bottom_height_;
 
+  // The height of the visible viewport in DPI-adjusted pixels.
+  int visible_viewport_height_;
+
   // The area that must be reserved for drawing the resize corner.
   gfx::Rect resizer_rect_;