Dirty rects always contain full tiles with delegated rendering.

cc/resources/bitmap_content_layer_updater.cc

Index: cc/resources/bitmap_content_layer_updater.cc
diff --git a/cc/resources/bitmap_content_layer_updater.cc b/cc/resources/bitmap_content_layer_updater.cc
index 7168c54f61e3141d1fce735780bbb2654f85b83c..fd0892353087e3c030e1f76fc22639d7ded97c43 100644
--- a/cc/resources/bitmap_content_layer_updater.cc
+++ b/cc/resources/bitmap_content_layer_updater.cc
@@ -55,6 +55,7 @@ scoped_ptr<LayerUpdater::Resource> BitmapContentLayerUpdater::CreateResource(
 
 void BitmapContentLayerUpdater::PrepareToUpdate(
     gfx::Rect content_rect,
+    gfx::Rect paint_rect,
     gfx::Size tile_size,
     float contents_width_scale,
     float contents_height_scale,
@@ -78,6 +79,7 @@ void BitmapContentLayerUpdater::PrepareToUpdate(
       rendering_stats_instrumentation_->StartRecording();
   PaintContents(canvas_.get(),
                 content_rect,
+                paint_rect,
                 contents_width_scale,
                 contents_height_scale,
                 resulting_opaque_rect);
@@ -94,7 +96,7 @@ void BitmapContentLayerUpdater::UpdateTexture(ResourceUpdateQueue* queue,
                                               gfx::Vector2d dest_offset,
                                               bool partial_update) {
   CHECK(canvas_);
-  gfx::Rect canvas_rect = content_rect();
+  gfx::Rect canvas_rect = paint_rect();
   canvas_rect.set_size(canvas_size_);
   ResourceUpdate upload = ResourceUpdate::CreateFromCanvas(
       texture, canvas_, canvas_rect, source_rect, dest_offset);