Derive the SourceAlpha filter input from SourceGraphic

Source/core/svg/graphics/filters/SVGFilterBuilder.h

Index: Source/core/svg/graphics/filters/SVGFilterBuilder.h
diff --git a/Source/core/svg/graphics/filters/SVGFilterBuilder.h b/Source/core/svg/graphics/filters/SVGFilterBuilder.h
index 3f09e694d35e0835584df648cf4bbd17cc321144..9328bded421d900ff1fc175cef478cd98472b778 100644
--- a/Source/core/svg/graphics/filters/SVGFilterBuilder.h
+++ b/Source/core/svg/graphics/filters/SVGFilterBuilder.h
@@ -37,9 +37,9 @@ class SVGFilterBuilder final : public RefCountedWillBeGarbageCollectedFinalized<
 public:
     typedef WillBeHeapHashSet<RawPtrWillBeMember<FilterEffect>> FilterEffectSet;
 
-    static PassRefPtrWillBeRawPtr<SVGFilterBuilder> create(PassRefPtrWillBeRawPtr<FilterEffect> sourceGraphic, PassRefPtrWillBeRawPtr<FilterEffect> sourceAlpha)
+    static PassRefPtrWillBeRawPtr<SVGFilterBuilder> create(PassRefPtrWillBeRawPtr<FilterEffect> sourceGraphic)
     {
-        return adoptRefWillBeNoop(new SVGFilterBuilder(sourceGraphic, sourceAlpha));
+        return adoptRefWillBeNoop(new SVGFilterBuilder(sourceGraphic));
     }
 
     void add(const AtomicString& id, PassRefPtrWillBeRawPtr<FilterEffect>);
@@ -65,7 +65,7 @@ public:
     DECLARE_TRACE();
 
 private:
-    SVGFilterBuilder(PassRefPtrWillBeRawPtr<FilterEffect> sourceGraphic, PassRefPtrWillBeRawPtr<FilterEffect> sourceAlpha);
+    SVGFilterBuilder(PassRefPtrWillBeRawPtr<FilterEffect> sourceGraphic);
 
     inline void addBuiltinEffects()
     {