cc: Add some early outs to avoid expensive operations

cc/occlusion_tracker.cc

Index: cc/occlusion_tracker.cc
diff --git a/cc/occlusion_tracker.cc b/cc/occlusion_tracker.cc
index 9c536d2e3a29e8977cdff3f1da09771eb5563edb..c5f7276e945cf4230930604ee8e3c34dc88f975a 100644
--- a/cc/occlusion_tracker.cc
+++ b/cc/occlusion_tracker.cc
@@ -349,6 +349,9 @@ bool OcclusionTrackerBase<LayerType, RenderSurfaceType>::occluded(const LayerTyp
 // the resulting unoccluded region is not rectangular, we return a rect containing it.
 static inline gfx::Rect rectSubtractRegion(const gfx::Rect& rect, const Region& region)
 {
+    if (region.IsEmpty())
+        return rect;
+
     Region rectRegion(rect);
     rectRegion.Subtract(region);
     return rectRegion.bounds();