Use demux API in SkWebpCodec

src/codec/SkCodec.cpp

 /*
  * 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 "SkCodec.h"
 #include "SkCodecPriv.h"
@@ skipping 124 matching lines @@
     , fOrigin(origin)
     , fDstInfo()
     , fOptions()
     , fCurrScanline(-1)
 {}
 
 SkCodec::~SkCodec() {}
 
 bool SkCodec::rewindIfNeeded() {
     if (!fStream) {
-        // Some codecs do not have a stream, but they hold others that do. They
-        // must handle rewinding themselves.
+        // Some codecs do not have a stream.  They may hold onto their own data or another codec.

[scroggo, 2016/09/07 17:33:04]

So when we pass the stream from webp, we're not doing it because the codec needs
it, but so we can hold onto it? 

We only keep it for a memory stream, so we'll unnecessarily rewind it, but it
will succeed.

That should all be fine then.

[msarett, 2016/09/07 17:36:30]

True.
Agreed.  Do you think there's a need to say more in the comments?

+        // They must handle rewinding themselves.
         return true;
     }
 
     // Store the value of fNeedsRewind so we can update it. Next read will
     // require a rewind.
     const bool needsRewind = fNeedsRewind;
     fNeedsRewind = true;
     if (!needsRewind) {
         return true;
     }
@@ skipping 236 matching lines @@
             SkASSERT(1 == linesRequested || this->getInfo().height() == linesRequested);
             const SkImageInfo fillInfo = info.makeWH(fillWidth, 1);
             for (int srcY = linesDecoded; srcY < linesRequested; srcY++) {
                 fillDst = SkTAddOffset<void>(dst, this->outputScanline(srcY) * rowBytes);
                 fill_proc(fillInfo, fillDst, rowBytes, fillValue, zeroInit, sampler);
             }
             break;
         }
     }
 }