Improve overdraw with multiple windows

cc/trees/occlusion_tracker_unittest.cc

 // Copyright 2012 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/occlusion_tracker.h"
 
 #include <stddef.h>
 
 #include "cc/animation/animation_host.h"
 #include "cc/base/filter_operation.h"
@@ skipping 1447 matching lines @@
       // needs should be removed from the occluded area, so that they are drawn
       // when we get to the parent.
       this->VisitContributingSurface(filtered_surface, &occlusion);
       this->EnterLayer(parent, &occlusion);
 
       gfx::Rect expected_occlusion;
       switch (i) {
         case LEFT:
           // The right half of the occlusion is close enough to cast a shadow
           // that would be visible in the background filter. The shadow reaches
-          // 3*5 + 10 = 25 pixels to the right.
-          expected_occlusion = gfx::Rect(0, 0, 25, 200);
+          // 3*5 + 10 = 25 pixels to the right, which is (0, 0, 25, 200). The
+          // rect created in filtered_surface is (50, 50, 100, 100), which
+          // becomes (25, 25, 20, 80) after applying gfx::Transform
+          // |scale_by_half|, adding FilterOperations |filters| and updating
+          // opacity pixel outside the rect. Union (0, 0, 25, 200) and
+          // (25, 25, 20, 80), it's (0, 25, 45, 80)
+          expected_occlusion = gfx::Rect(0, 25, 45, 80);
           break;
         case RIGHT:
           // The shadow spreads 3*5 - 10 = 5 pixels to the left, so the
           // occlusion must recede by 5 to account for that.
           expected_occlusion = gfx::Rect(105, 0, 45, 200);
           break;
         case TOP:
           // Similar to LEFT.
-          expected_occlusion = gfx::Rect(0, 0, 200, 25);
+          expected_occlusion = gfx::Rect(25, 0, 80, 45);
           break;
         case BOTTOM:
           // Similar to RIGHT.
           expected_occlusion = gfx::Rect(0, 105, 200, 45);
           break;
       }
 
       EXPECT_EQ(expected_occlusion.ToString(),
                 occlusion.occlusion_from_inside_target().ToString());
       EXPECT_TRUE(occlusion.occlusion_from_outside_target().IsEmpty());
@@ skipping 944 matching lines @@
     EXPECT_EQ(gfx::Rect(),
               occlusion.UnoccludedSurfaceContentRect(
                   surface, gfx::Rect(80, 70, 50, 50)));
   }
 };
 
 ALL_OCCLUSIONTRACKER_TEST(OcclusionTrackerTestUnoccludedSurfaceQuery)
 
 }  // namespace
 }  // namespace cc