Call rewindIfNeeded in SkCodec

Rather than calling it in each subclass, call it once in the base
class. Call it first, since other steps may modify internal structures
which would be replaced by a call to onRewind.

BUG=skia:4284

Review URL: https://codereview.chromium.org/1381483002
diff --git a/include/codec/SkCodec.h b/include/codec/SkCodec.h
index 01fd4fa..c0cce85 100644
--- a/include/codec/SkCodec.h
+++ b/include/codec/SkCodec.h
@@ -402,9 +402,8 @@
      *  @returns true if the codec is at the right position and can be used.
      *      false if there was a failure to rewind.
      *
-     *  Subclasses MUST call this function before reading the stream (e.g. in
-     *  onGetPixels). If it returns false, onGetPixels should return
-     *  kCouldNotRewind.
+     *  This is called by getPixels() and start(). Subclasses may call if they
+     *  need to rewind at another time.
      */
     bool SK_WARN_UNUSED_RESULT rewindIfNeeded();