2D canvas: fix shadow rendering with compositing modes that require temp layers

Source/platform/graphics/GraphicsContext.cpp

 /*
  * Copyright (C) 2003, 2004, 2005, 2006, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 281 matching lines @@
     if (!gradient) {
         setFillColor(Color::black);
         return;
     }
 
     mutableState()->setFillGradient(gradient);
 }
 
 void GraphicsContext::setShadow(const FloatSize& offset, float blur, const Color& color,
     DrawLooperBuilder::ShadowTransformMode shadowTransformMode,
-    DrawLooperBuilder::ShadowAlphaMode shadowAlphaMode)
+    DrawLooperBuilder::ShadowAlphaMode shadowAlphaMode, ShadowMode shadowMode)
 {
     if (contextDisabled())
         return;
 
+    OwnPtr<DrawLooperBuilder> drawLooperBuilder = DrawLooperBuilder::create();
     if (!color.alpha() || (!offset.width() && !offset.height() && !blur)) {
+        if (shadowMode == DrawShadowOnly) {
+            // shadow only, but there is no shadow: use an empty draw looper to silence rendering.

[Stephen White, 2014/10/15 17:15:30]

Nit: suggest "silence rendering" -> "disable rendering of the source primitive".

+            setDrawLooper(drawLooperBuilder.release());
+            return;
+        }
         clearShadow();
         return;
     }
 
-    OwnPtr<DrawLooperBuilder> drawLooperBuilder = DrawLooperBuilder::create();
     drawLooperBuilder->addShadow(offset, blur, color, shadowTransformMode, shadowAlphaMode);
-    drawLooperBuilder->addUnmodifiedContent();
+    if (shadowMode == DrawShadowAndForeground) {
+        drawLooperBuilder->addUnmodifiedContent();
+    }
     setDrawLooper(drawLooperBuilder.release());
 }
 
 void GraphicsContext::setDrawLooper(PassOwnPtr<DrawLooperBuilder> drawLooperBuilder)
 {
     if (contextDisabled())
         return;
 
     mutableState()->setDrawLooper(drawLooperBuilder->detachDrawLooper());
 }
@@ skipping 1688 matching lines @@
         // FIXME: This is to not break tests (it results in the filter bitmap flag
         // being set to true). We need to decide if we respect InterpolationNone
         // being returned from computeInterpolationQuality.
         resampling = InterpolationLow;
     }
     resampling = limitInterpolationQuality(this, resampling);
     paint->setFilterLevel(static_cast<SkPaint::FilterLevel>(resampling));
 }
 
 } // namespace blink