Send array of paint rects and bitmaps as opposed to a Union

chrome/common/render_messages.h

Index: chrome/common/render_messages.h
===================================================================
--- chrome/common/render_messages.h	(revision 17347)
+++ chrome/common/render_messages.h	(working copy)
@@ -158,12 +158,17 @@
 };
 
 struct ViewHostMsg_PaintRect_Params {
-  // The bitmap to be painted into the rect given by bitmap_rect.
+  // The bitmap to be painted into the rects given by paint_rects.
   TransportDIB::Id bitmap;
 
-  // The position and size of the bitmap.
+  // The position and size of the the bitmap relative to the view port.
   gfx::Rect bitmap_rect;
 
+  // The position and size of the rects where the bitmap has data to be
+  // painted to the view. An empty vector means that the whole bitmap has
+  // data to be painted to the view.
+  std::vector<gfx::Rect> paint_rects;
+
   // The size of the RenderView when this message was generated.  This is
   // included so the host knows how large the view is from the perspective of
   // the renderer process.  This is necessary in case a resize operation is in
@@ -858,6 +863,7 @@
   static void Write(Message* m, const param_type& p) {
     WriteParam(m, p.bitmap);
     WriteParam(m, p.bitmap_rect);
+    WriteParam(m, p.paint_rects);
     WriteParam(m, p.view_size);
     WriteParam(m, p.plugin_window_moves);
     WriteParam(m, p.flags);
@@ -866,6 +872,7 @@
     return
       ReadParam(m, iter, &p->bitmap) &&
       ReadParam(m, iter, &p->bitmap_rect) &&
+      ReadParam(m, iter, &p->paint_rects) &&
       ReadParam(m, iter, &p->view_size) &&
       ReadParam(m, iter, &p->plugin_window_moves) &&
       ReadParam(m, iter, &p->flags);
@@ -876,6 +883,8 @@
     l->append(L", ");
     LogParam(p.bitmap_rect, l);
     l->append(L", ");
+    LogParam(p.paint_rects, l);
+    l->append(L", ");
     LogParam(p.view_size, l);
     l->append(L", ");
     LogParam(p.plugin_window_moves, l);