Add a static method to WebPDecoder to decode WebP

ios/web/public/image_fetcher/webp_decoder.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef IOS_WEB_PUBLIC_IMAGE_FETCHER_WEBP_DECODER_H_
 #define IOS_WEB_PUBLIC_IMAGE_FETCHER_WEBP_DECODER_H_
 
 #import <Foundation/Foundation.h>
 #include <stddef.h>
 
@@ skipping 22 matching lines @@
                                   DecodedImageFormat format) = 0;
     virtual void OnDataDecoded(NSData* data) = 0;
 
    protected:
     friend class base::RefCountedThreadSafe<WebpDecoder::Delegate>;
     virtual ~Delegate() {}
   };
 
   explicit WebpDecoder(WebpDecoder::Delegate* delegate);
 
+  // Returns an NSData object containing the decoded image data of the given
+  // webp_image. Returns nil in case of failure.
+  static NSData* DecodeWebpImage(NSData* webp_image);
+
+  // Returns true if the given image_data is a WebP image.
+  //
+  // Every WebP file contains a 12 byte file header in the beginning of the
+  // file.
+  // A WebP file header starts with the four ASCII characters "RIFF". The next
+  // four bytes contain the image size and the last four header bytes contain
+  // the four ASCII characters "WEBP".
+  //
+  // WebP file header:
+  //                                  1 1
+  // Byte Nr.     0 1 2 3 4 5 6 7 8 9 0 1
+  // Byte value [ R I F F ? ? ? ? W E B P  ]
+  //
+  // For more information see:
+  // https://developers.google.com/speed/webp/docs/riff_container#webp_file_header
+  static bool IsWebpImage(const std::string& image_data);
+
   // For tests.
   static size_t GetHeaderSize();
 
   // Main entry point.
   void OnDataReceived(NSData* data);
 
   // Stops the decoding.
   void Stop();
 
  private:
@@ skipping 16 matching lines @@
   WebpDecoder::State state_;
   std::unique_ptr<WebPIDecoder, WebPIDecoderDeleter> incremental_decoder_;
   base::scoped_nsobject<NSData> output_buffer_;
   base::scoped_nsobject<NSMutableData> features_;
   int has_alpha_;
 };
 
 }  // namespace webp_transcode
 
 #endif  // IOS_WEB_PUBLIC_IMAGE_FETCHER_WEBP_DECODER_H_