cc: early return if unoccluded_region_in_target_surface is empty

cc/trees/occlusion_tracker.cc

Index: cc/trees/occlusion_tracker.cc
diff --git a/cc/trees/occlusion_tracker.cc b/cc/trees/occlusion_tracker.cc
index f4a414f6de29c35815f8abf9592974f0c56a6e1a..dbba04e8740334661a4d4c1ba860c24008383879 100644
--- a/cc/trees/occlusion_tracker.cc
+++ b/cc/trees/occlusion_tracker.cc
@@ -567,6 +567,9 @@ gfx::Rect OcclusionTracker<LayerType>::UnoccludedContentRect(
   unoccluded_region_in_target_surface.Subtract(
       stack_.back().occlusion_from_outside_target);
 
+  if (unoccluded_region_in_target_surface.IsEmpty())

[danakj, 2014/07/14 15:14:53]

ProjectEnclosingClippedRect and Intersect both have early outs that would be hit
in this case. Is adding another early out here a win? Did you measure this in
some way?

[hyunki, 2014/07/14 15:29:34]

I think Intersect have early out.
However, when I checked ProjectEnclosingClippedRect, it checks transform only 
and doesn't check source rect (unoccluded_rect_in_target_surface), thus I think
it doesn't out early.   

[danakj, 2014/07/14 16:00:08]

Oh, right sorry. Ok sounds good, does this have any impact on the
occlusion_tracker_perftests?

[hyunki, 2014/07/14 16:25:09]

When I added logs, I've checked it falls early outs for many cases. (perftest
and unittest both)
perftest doesn't give big improvements on my machine as belows.

--> Before
[1/2] OcclusionTrackerPerfTest.UnoccludedContentRect_FullyOccluded (2005 ms)
[2/2] OcclusionTrackerPerfTest.UnoccludedContentRect_10OpaqueLayers (2005 ms)
--> After
[1/2] OcclusionTrackerPerfTest.UnoccludedContentRect_FullyOccluded (1999 ms)
[2/2] OcclusionTrackerPerfTest.UnoccludedContentRect_10OpaqueLayers (2000 ms)

[danakj, 2014/07/14 16:26:02]

It's not testing this case, so I would expect no change or a regression. No
change is good.

+    return gfx::Rect();
+
   gfx::Rect unoccluded_rect_in_target_surface =
       unoccluded_region_in_target_surface.bounds();
   gfx::Rect unoccluded_rect = MathUtil::ProjectEnclosingClippedRect(