Add the ability to decode a subset to SkCodec.

src/codec/SkWebpCodec.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkWebpCodec_DEFINED
 #define SkWebpCodec_DEFINED
 
@@ skipping 12 matching lines @@
 protected:
     Result onGetPixels(const SkImageInfo&, void*, size_t, const Options&, SkPMColor*, int*)
             override;
     SkEncodedFormat onGetEncodedFormat() const override { return kWEBP_SkEncodedFormat; }
 
     bool onReallyHasAlpha() const override {
         return this->getInfo().alphaType() != kOpaque_SkAlphaType;
     }
 
     SkISize onGetScaledDimensions(float desiredScale) const override;
+
+    bool onGetValidSubset(SkIRect* /* desiredSubset */) const override;
 private:
     SkWebpCodec(const SkImageInfo&, SkStream*);
 
     typedef SkCodec INHERITED;
 };
 #endif // SkWebpCodec_DEFINED