SkValue: improve SkFromValue<T> implementation

src/core/SkValue.h

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkValue_DEFINED
 #define SkValue_DEFINED
 
@@ skipping 55 matching lines @@
     uint32_t u32() const;
     float    f32() const;
     SkData* bytes() const;
 
     const uint16_t* u16s(int* count) const;
     const uint32_t* u32s(int* count) const;
     const float*    f32s(int* count) const;
 
     // Object
     void set(Key, SkValue);
+    const SkValue* get(Key) const;
     void foreach(std::function<void(Key, const SkValue&)>) const;
 
     // Array
     size_t length() const;
     const SkValue& at(size_t) const;
     void append(SkValue);
 
 private:
     class Obj;
     class Arr;
@@ skipping 21 matching lines @@
     template <typename T> const T* asTs(SkValue::Type, int*) const;
 };
 
 template <typename T>
 SkValue SkToValue(const T&);
 
 template <typename T>
 bool SkFromValue(const SkValue&, T*);
 
 #endif  // SkValue_DEFINED