Add some backend flexibility for shader declarations in ProgramBuilding.

src/gpu/glsl/GrGLSLVarying.h

Index: src/gpu/glsl/GrGLSLVarying.h
diff --git a/src/gpu/glsl/GrGLSLVarying.h b/src/gpu/glsl/GrGLSLVarying.h
index 116ba09b99bd9d2b797a328494bd49b903d5ca81..22431978c859666d57d33dcc0619c51694420c0e 100644
--- a/src/gpu/glsl/GrGLSLVarying.h
+++ b/src/gpu/glsl/GrGLSLVarying.h
@@ -79,6 +79,8 @@ public:
         , fFragOutputs(kVaryingsPerBlock)
         , fProgramBuilder(program) {}
 
+    virtual ~GrGLSLVaryingHandler() {}
+
     typedef GrTAllocator<GrGLSLShaderVar> VarArray;
     typedef GrGLSLProgramDataManager::VaryingHandle VaryingHandle;
 
@@ -106,6 +108,10 @@ public:
 
     void emitAttributes(const GrGeometryProcessor& gp);
 
+    // This should be called once all attributes and varyings have been added to the
+    // GrGLSLVaryingHanlder and before getting/adding any of the declarations to the shaders.
+    void finalize();
+
     void getVertexDecls(SkString* inputDecls, SkString* outputDecls) const;
     void getGeomDecls(SkString* inputDecls, SkString* outputDecls) const;
     void getFragDecls(SkString* inputDecls, SkString* outputDecls) const;
@@ -127,6 +133,8 @@ private:
 
     void addAttribute(const GrShaderVar& var);
 
+    virtual void onFinalize() = 0;
+
     // helper function for get*Decls
     void appendDecls(const VarArray& vars, SkString* out) const;