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 269 matching lines @@
   // ancestor surface, ExtendDamageForRenderSurface() must be called instead.
 
   bool layer_is_new = false;
   RectMapData& data = RectDataForLayer(layer->id(), &layer_is_new);
   gfx::Rect old_rect_in_target_space = data.rect_;
 
   gfx::Rect rect_in_target_space = MathUtil::MapEnclosingClippedRect(
       layer->draw_transform(), gfx::Rect(layer->content_bounds()));
   data.Update(rect_in_target_space, mailboxId_);
 
+  // Union layer's update_rect with damage_rect

[reveman, 2014/04/15 16:25:59]

nit: missing period at the end of the line.

+  gfx::RectF unified_update_and_damage_rect =

[reveman, 2014/04/15 16:25:59]

The comment and variable names below are no longer properly named. I would just
call this damage_rect, make it clear in the comment above that this is the union
of update_/content_damage_rect and make the comment and variable names below
consistent with that.

+      gfx::UnionRects(layer->update_rect(), layer->damage_content_rect());
+
   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()) {
+  } else if (!unified_update_and_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());
+        layer->LayerRectToContentRect(unified_update_and_damage_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);
   }
 }
 
 void DamageTracker::ExtendDamageForRenderSurface(
     LayerImpl* layer,
     gfx::Rect* target_damage_rect) {
   // There are two ways a "descendant surface" can damage regions of the "target
@@ skipping 81 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