Add canvas blending modes under a runtime flag

Source/core/html/canvas/CanvasRenderingContext2D.cpp

Index: Source/core/html/canvas/CanvasRenderingContext2D.cpp
diff --git a/Source/core/html/canvas/CanvasRenderingContext2D.cpp b/Source/core/html/canvas/CanvasRenderingContext2D.cpp
index 12d51b653e0e34c11363644220f8d67f40254ea9..c2af5ca693e98930f7ca4beb0006e4f1a9776987 100644
--- a/Source/core/html/canvas/CanvasRenderingContext2D.cpp
+++ b/Source/core/html/canvas/CanvasRenderingContext2D.cpp
@@ -35,6 +35,7 @@
 
 #include "CSSPropertyNames.h"
 #include "HTMLNames.h"
+#include "RuntimeEnabledFeatures.h"
 #include "core/css/CSSFontSelector.h"
 #include "core/css/CSSParser.h"
 #include "core/css/StylePropertySet.h"
@@ -611,6 +612,8 @@ void CanvasRenderingContext2D::setGlobalCompositeOperation(const String& operati
     BlendMode blendMode = BlendModeNormal;
     if (!parseCompositeAndBlendOperator(operation, op, blendMode))
         return;
+    if (!RuntimeEnabledFeatures::cssCompositingEnabled() && blendMode != BlendModeNormal)
+        blendMode = BlendModeNormal;
     if ((state().m_globalComposite == op) && (state().m_globalBlend == blendMode))
         return;
     realizeSaves();