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..07f090479d9a18b5a6e5731ad119ba19223a00a2 100644
--- a/Source/platform/graphics/GraphicsContext.cpp
+++ b/Source/platform/graphics/GraphicsContext.cpp
@@ -299,19 +299,26 @@ 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;
 
+    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 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());
 }