Canvas2d: Make the intersection computation O(1) instead of O(n) in ClipList.

third_party/WebKit/Source/modules/canvas2d/CanvasRenderingContext2D.cpp

 /*
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2007 Alp Toker <alp@atoker.com>
  * Copyright (C) 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2008 Dirk Schulze <krit@webkit.org>
  * Copyright (C) 2010 Torch Mobile (Beijing) Co. Ltd. All rights reserved.
  * Copyright (C) 2012, 2013 Intel Corporation. All rights reserved.
  * Copyright (C) 2013 Adobe Systems Incorporated. All rights reserved.
  *
@@ skipping 2244 matching lines @@
 
     if (hitRegionPath.isEmpty() || !c || !state().isTransformInvertible()
         || !c->getClipDeviceBounds(0)) {
         exceptionState.throwDOMException(NotSupportedError, "The specified path has no pixels.");
         return;
     }
 
     hitRegionPath.transform(state().transform());
 
     if (state().hasClip()) {
-        hitRegionPath = state().intersectPathWithClip(hitRegionPath.skPath());
+        hitRegionPath.intersectPath(state().getCurrentClipPath());
         if (hitRegionPath.isEmpty())
             exceptionState.throwDOMException(NotSupportedError, "The specified path has no pixels.");
     }
 
     if (!m_hitRegionManager)
         m_hitRegionManager = HitRegionManager::create();
 
     // Remove previous region (with id or control)
     m_hitRegionManager->removeHitRegionById(options.id());
     m_hitRegionManager->removeHitRegionByControl(options.control().get());
@@ skipping 94 matching lines @@
         if (imageType == CanvasRenderingContext2DState::NonOpaqueImage)
             return;
         if (alpha < 0xFF)
             return;
     }
 
     canvas()->buffer()->willOverwriteCanvas();
 }
 
 } // namespace blink