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

cc/trees/damage_tracker.cc

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/trees/damage_tracker.h"
 
 #include <algorithm>
 
 #include "cc/base/math_util.h"
 #include "cc/layers/heads_up_display_layer_impl.h"
@@ skipping 271 matching lines @@
 
   bool layer_is_new = false;
   RectMapData& data = RectDataForLayer(layer->id(), &layer_is_new);
   gfx::RectF old_rect_in_target_space = data.rect_;
 
   gfx::RectF rect_in_target_space = MathUtil::MapClippedRect(
       layer->draw_transform(),
       gfx::RectF(gfx::PointF(), layer->content_bounds()));
   data.Update(rect_in_target_space, mailboxId_);
 
+  // Union layer's update_rect with damage_rect
+  if (!layer->damage_rect().IsEmpty()) {
+    gfx::RectF rect = layer->update_rect();
+    gfx::RectF layer_rect = gfx::ScaleRect(layer->damage_rect(),
+                                           1 / layer->contents_scale_x(),
+                                           1 / layer->contents_scale_y());
+    layer->SetUpdateRect(gfx::UnionRects(rect, layer_rect));

[danakj, 2014/04/14 17:17:07]

instead of setting the UpdateRect on the layer, just do the union of the damage
rect and the update rect here locally. Changing the UpdateRect here will mess
with later debug visualizations that collect them.

[sohanjg, 2014/04/15 10:03:48]

Done.

+    layer->ResetDamageRect();

[danakj, 2014/04/14 17:17:07]

Instead of clearing the damage rect here, you can clear it in
LayerImpl::ResetAllChangeTrackingForSubtree.

[sohanjg, 2014/04/15 10:03:48]

Done.

+  }
+
   if (layer_is_new || layer->LayerPropertyChanged()) {
     // If a layer is new or has changed, then its entire layer rect affects the
     // target surface.
     target_damage_rect->Union(rect_in_target_space);
 
     // 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()) {
     // If the layer properties haven't changed, then the the target surface is
@@ skipping 94 matching lines @@
   // one in them. This means we need to redraw any pixels in the surface being
   // used for the blur in this layer this frame.
   if (layer->background_filters().HasFilterThatMovesPixels()) {
     ExpandDamageRectInsideRectWithFilters(target_damage_rect,
                                           surface_rect_in_target_space,
                                           layer->background_filters());
   }
 }
 
 }  // namespace cc