Implement onSkipScanlines() for bmp and wbmp

src/codec/SkBmpMaskCodec.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.
  */
 
 #include "SkBmpCodec.h"
 #include "SkImageInfo.h"
 #include "SkMaskSwizzler.h"
@@ skipping 37 matching lines @@
     SkSampler* getSampler(bool createIfNecessary) override {
         SkASSERT(fMaskSwizzler);
         return fMaskSwizzler;
     }
 
     int decodeRows(const SkImageInfo& dstInfo, void* dst, size_t dstRowBytes,
             const Options& opts) override;
 
     SkAutoTDelete<SkMasks>              fMasks;        // owned
     SkAutoTDelete<SkMaskSwizzler>       fMaskSwizzler;
-    const size_t                        fSrcRowBytes;
     SkAutoTDeleteArray<uint8_t>         fSrcBuffer;
 
     typedef SkBmpCodec INHERITED;
 };