Handle SkStream::rewind properly.

include/core/SkStream.h

Index: include/core/SkStream.h
diff --git a/include/core/SkStream.h b/include/core/SkStream.h
index d7a105d3b84e1080fdc4157b1d40f52a1c68750c..e68901049d7b2b5c6666785dfead888bf890ce6a 100644
--- a/include/core/SkStream.h
+++ b/include/core/SkStream.h
@@ -96,7 +96,13 @@ public:
     SkData* readData();
 
 //SkStreamRewindable
-    /** Rewinds to the beginning of the stream. If this cannot be done, return false. */
+    /** Rewinds to the beginning of the stream. The current position is not affected
+     *  when false is returned.
+     *
+     *  Returns false iff after this call one of the following is true:
+     *      * The stream is known to be somewhere other than the beginning.
+     *      * It is unknown whether the stream is at the beginning.

[reed1, 2013/08/27 20:00:37]

Are both of these iff clauses true at the same times as "the position is not
affected" is true?

[scroggo, 2013/08/27 20:12:56]

No. An implementation could return false if the stream was already at the
beginning - this would satisfy the condition of the initial statement, but not
the iff clause.

Our decoders now fail if rewind returned false, so I want to make it clear to
anyone writing an implementation that they should not return false if the stream
was already at the beginning (which should not cause a failure to decode). The
previous comment leaves open to interpretation what it means to be able to
rewind, IMO.

+     */
     virtual bool rewind() { return false; }
 
     /** Duplicates this stream. If this cannot be done, returns NULL.