cc: Add support for partial swaps when using impl-side painting.

cc/trees/damage_tracker.cc

Index: cc/trees/damage_tracker.cc
diff --git a/cc/trees/damage_tracker.cc b/cc/trees/damage_tracker.cc
index 40741461320fa0affe36e8f7d8628d61691b1271..866f09d65692388c8e508d3456ed35054dd49d1b 100644
--- a/cc/trees/damage_tracker.cc
+++ b/cc/trees/damage_tracker.cc
@@ -287,6 +287,9 @@ void DamageTracker::ExtendDamageForLayer(LayerImpl* layer,
       layer->draw_transform(), gfx::Rect(layer->content_bounds()));
   data.Update(rect_in_target_space, mailboxId_);
 
+  gfx::RectF damage_rect =
+      gfx::UnionRects(layer->update_rect(), layer->damage_rect());
+
   if (layer_is_new || layer->LayerPropertyChanged()) {
     // If a layer is new or has changed, then its entire layer rect affects the
     // target surface.
@@ -295,14 +298,13 @@ void DamageTracker::ExtendDamageForLayer(LayerImpl* layer,
     // The layer's old region is now exposed on the target surface, too.
     // Note old_rect_in_target_space is already in target space.
     target_damage_rect->Union(old_rect_in_target_space);
-  } else if (!layer->update_rect().IsEmpty()) {
+  } else if (!damage_rect.IsEmpty()) {
     // If the layer properties haven't changed, then the the target surface is
-    // only affected by the layer's update area, which could be empty.
-    gfx::Rect update_content_rect =
-        layer->LayerRectToContentRect(layer->update_rect());
-    gfx::Rect update_rect_in_target_space = MathUtil::MapEnclosingClippedRect(
-        layer->draw_transform(), update_content_rect);
-    target_damage_rect->Union(update_rect_in_target_space);
+    // only affected by the layer's damaged area, which could be empty.
+    gfx::Rect damage_content_rect = layer->LayerRectToContentRect(damage_rect);
+    gfx::Rect damage_rect_in_target_space = MathUtil::MapEnclosingClippedRect(
+        layer->draw_transform(), damage_content_rect);
+    target_damage_rect->Union(damage_rect_in_target_space);
   }
 }