Make SkCodec support peek() and read()

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
 
 #include "SkCodec.h"
 #include "SkEncodedFormat.h"
 #include "SkImageInfo.h"
 #include "SkTypes.h"
 
 class SkStream;
 
+static const size_t WEBP_VP8_HEADER_SIZE = 30;
+
 class SkWebpCodec final : public SkCodec {
 public:
     // Assumes IsWebp was called and returned true.
     static SkCodec* NewFromStream(SkStream*);
-    static bool IsWebp(SkStream*);
+    static bool IsWebp(const char*, size_t);
 protected:
     Result onGetPixels(const SkImageInfo&, void*, size_t, const Options&, SkPMColor*, int*, 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 onDimensionsSupported(const SkISize&) override;
 
     bool onGetValidSubset(SkIRect* /* desiredSubset */) const override;
 private:
     SkWebpCodec(const SkImageInfo&, SkStream*);
 
     typedef SkCodec INHERITED;
 };
 #endif // SkWebpCodec_DEFINED