Use demux API in SkWebpCodec

src/codec/SkCodec.cpp

Index: src/codec/SkCodec.cpp
diff --git a/src/codec/SkCodec.cpp b/src/codec/SkCodec.cpp
index 3192a6f4be8ad6127d1e1ba63f21d70f976ba6ff..2372f4ce5b04457a921d0a82f4551f49e73c0b95 100644
--- a/src/codec/SkCodec.cpp
+++ b/src/codec/SkCodec.cpp
@@ -142,8 +142,8 @@ 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;
     }