Simplify SkPDFShader class. Now invalid objects are never created.

src/pdf/SkPDFShader.h

Index: src/pdf/SkPDFShader.h
diff --git a/src/pdf/SkPDFShader.h b/src/pdf/SkPDFShader.h
index e0abff8f68388324128cbe15bfebfc4d49540bc2..10532156ad20b77d742b618154d56e25f599e53f 100644
--- a/src/pdf/SkPDFShader.h
+++ b/src/pdf/SkPDFShader.h
@@ -54,14 +54,11 @@ protected:
     // This is an internal method.
     // CanonicalShadersMutex() should already be acquired.
     // This also takes ownership of shaderState.
-    static SkPDFObject* GetPDFShaderByState(State* shaderState);
-    static SkPDFObject* AddToCanonIfValid(SkPDFShader*);
-    static void RemoveFromCanonIfValid(SkPDFShader*);
+    static SkPDFObject* GetPDFShaderByState(SkAutoTDelete<SkPDFShader::State>*);
 
     SkPDFShader(State*);
     virtual ~SkPDFShader();
 
-    virtual bool isValid() = 0;
     virtual SkPDFObject* toPDFObject() = 0;
 };