cc: Add some early outs to avoid expensive operations

cc/occlusion_tracker.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 "config.h"
 
 #include "cc/occlusion_tracker.h"
 
 #include <algorithm>
 
@@ skipping 331 matching lines @@
         return true;
     }
 
     return false;
 }
 
 // Determines what portion of rect, if any, is unoccluded (not occluded by region). If
 // 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();
 }
 
 static inline gfx::Rect computeUnoccludedContentRect(const gfx::Rect& contentRect, const WebTransformationMatrix& contentSpaceTransform, const gfx::Rect& clipRectInTarget, const Region& occlusion)
 {
     if (!contentSpaceTransform.isInvertible())
         return contentRect;
 
@@ skipping 96 matching lines @@
     // FIXME: we could remove this helper function, but unit tests currently override this
     //        function, and they need to be verified/adjusted before this can be removed.
     return layer->drawableContentRect();
 }
 
 // Instantiate (and export) templates here for the linker.
 template class OcclusionTrackerBase<Layer, RenderSurface>;
 template class OcclusionTrackerBase<LayerImpl, RenderSurfaceImpl>;
 
 }  // namespace cc