Derive the SourceAlpha filter input from SourceGraphic

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

Index: Source/core/svg/graphics/filters/SVGFilterBuilder.cpp
diff --git a/Source/core/svg/graphics/filters/SVGFilterBuilder.cpp b/Source/core/svg/graphics/filters/SVGFilterBuilder.cpp
index 5c3104812fcc423ebac0aab967cda1757de4d8b6..b861321d9555ea7833b840314821a395f3f278d5 100644
--- a/Source/core/svg/graphics/filters/SVGFilterBuilder.cpp
+++ b/Source/core/svg/graphics/filters/SVGFilterBuilder.cpp
@@ -25,10 +25,11 @@
 
 namespace blink {
 
-SVGFilterBuilder::SVGFilterBuilder(PassRefPtrWillBeRawPtr<FilterEffect> sourceGraphic, PassRefPtrWillBeRawPtr<FilterEffect> sourceAlpha)
+SVGFilterBuilder::SVGFilterBuilder(PassRefPtrWillBeRawPtr<FilterEffect> prpSourceGraphic)
 {
+    RefPtrWillBeRawPtr<FilterEffect> sourceGraphic = prpSourceGraphic;

[Stephen White, 2015/04/20 17:04:49]

Nit: Blink coding style prefers no abbreviations. Suggest the parameter stay
sourceGraphic, and the local become sourceGraphicRef, or something like that.

[fs, 2015/04/21 10:29:20]

Done.

     m_builtinEffects.add(SourceGraphic::effectName(), sourceGraphic);
-    m_builtinEffects.add(SourceAlpha::effectName(), sourceAlpha);
+    m_builtinEffects.add(SourceAlpha::effectName(), SourceAlpha::create(sourceGraphic.get()));
     addBuiltinEffects();
 }