Add canvas blending modes under a runtime flag

Source/WebCore/platform/graphics/skia/ImageSkia.cpp

Index: Source/WebCore/platform/graphics/skia/ImageSkia.cpp
===================================================================
--- Source/WebCore/platform/graphics/skia/ImageSkia.cpp	(revision 147814)
+++ Source/WebCore/platform/graphics/skia/ImageSkia.cpp	(working copy)
@@ -617,7 +617,7 @@
     draw(ctxt, dstRect, srcRect, colorSpace, compositeOp, blendMode, DoNotRespectImageOrientation);
 }
 
-void BitmapImage::draw(GraphicsContext* ctxt, const FloatRect& dstRect, const FloatRect& srcRect, ColorSpace colorSpace, CompositeOperator compositeOp, BlendMode, RespectImageOrientationEnum shouldRespectImageOrientation)
+void BitmapImage::draw(GraphicsContext* ctxt, const FloatRect& dstRect, const FloatRect& srcRect, ColorSpace colorSpace, CompositeOperator compositeOp, BlendMode blendMode, RespectImageOrientationEnum shouldRespectImageOrientation)
 {
     // Spin the animation to the correct frame before we try to draw it, so we
     // don't draw an old frame and then immediately need to draw a newer one,
@@ -660,7 +660,7 @@
         *bm,
         normSrcRect,
         normDstRect,
-        WebCoreCompositeToSkiaComposite(compositeOp));
+        WebCoreCompositeToSkiaComposite(compositeOp, blendMode));
 
     if (ImageObserver* observer = imageObserver())
         observer->didDraw(this);