Handle SkStream::rewind properly.

src/images/SkImageDecoder_libwebp.cpp

Index: src/images/SkImageDecoder_libwebp.cpp
diff --git a/src/images/SkImageDecoder_libwebp.cpp b/src/images/SkImageDecoder_libwebp.cpp
index 9f1116e5d5bbbd51c8e168f12d8db99d585f07f6..7fb1cf0e312194e9f5c17ffb6fc4d0806adc9886 100644
--- a/src/images/SkImageDecoder_libwebp.cpp
+++ b/src/images/SkImageDecoder_libwebp.cpp
@@ -199,7 +199,10 @@ static bool webp_idecode(SkStream* stream, WebPDecoderConfig* config) {
         return false;
     }
 
-    stream->rewind();
+    if (!stream->rewind()) {
+        SkDebugf("Failed to rewind webp stream!");
+        return false;
+    }
     const size_t readBufferSize = stream->hasLength() ?
             SkTMin(stream->getLength(), WEBP_IDECODE_BUFFER_SZ) : WEBP_IDECODE_BUFFER_SZ;
     SkAutoMalloc srcStorage(readBufferSize);
@@ -311,7 +314,11 @@ bool SkWEBPImageDecoder::onBuildTileIndex(SkStream* stream,
         return false;
     }
 
-    stream->rewind();
+    if (!stream->rewind()) {
+        SkDebugf("Failed to rewind webp stream!");
+        return false;
+    }
+
     *width = origWidth;
     *height = origHeight;