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

Source/platform/graphics/GraphicsContext.cpp

Index: Source/platform/graphics/GraphicsContext.cpp
diff --git a/Source/platform/graphics/GraphicsContext.cpp b/Source/platform/graphics/GraphicsContext.cpp
index 1fdbb54a9eca53d367f662b2f557efa68680b116..c431e280370db130862470e507829647fe6fa26a 100644
--- a/Source/platform/graphics/GraphicsContext.cpp
+++ b/Source/platform/graphics/GraphicsContext.cpp
@@ -299,22 +299,31 @@ void GraphicsContext::setFillGradient(PassRefPtr<Gradient> 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;
 
-    if (!color.alpha() || (!offset.width() && !offset.height() && !blur)) {
+    OwnPtr<DrawLooperBuilder> drawLooperBuilder = DrawLooperBuilder::create();
+    if (shadowMode == DrawForegroundOnly || (!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, 2014/10/14 21:02:12]

Not sure I understand this comment. "silence rendering"? Why do we need a draw
looper at all in this case?

+            setDrawLooper(drawLooperBuilder.release());
+            return;
+        }
+        // Drawing foreground only, or drawing shadow and forground with no shadow.
         clearShadow();
         return;
     }
 
-    OwnPtr<DrawLooperBuilder> drawLooperBuilder = DrawLooperBuilder::create();
     drawLooperBuilder->addShadow(offset, blur, color, shadowTransformMode, shadowAlphaMode);
-    drawLooperBuilder->addUnmodifiedContent();
+    if (shadowMode == DrawShadowAndForeground) {
+        drawLooperBuilder->addUnmodifiedContent();
+    }
     setDrawLooper(drawLooperBuilder.release());
 }
 
+

[jbroman, 2014/10/14 21:09:48]

nit: spurious newline

 void GraphicsContext::setDrawLooper(PassOwnPtr<DrawLooperBuilder> drawLooperBuilder)
 {
     if (contextDisabled())