libwebp: update to v0.5.1

third_party/libwebp/dec/vp8li.h

Index: third_party/libwebp/dec/vp8li.h
diff --git a/third_party/libwebp/dec/vp8li.h b/third_party/libwebp/dec/vp8li.h
index 8886e47f62d9d8ef620af92a08d5ed69a0508212..9313bdc0af97f0e21433bd83e5f8fbdbb263320f 100644
--- a/third_party/libwebp/dec/vp8li.h
+++ b/third_party/libwebp/dec/vp8li.h
@@ -100,8 +100,7 @@ struct ALPHDecoder;  // Defined in dec/alphai.h.
 // Decodes image header for alpha data stored using lossless compression.
 // Returns false in case of error.
 int VP8LDecodeAlphaHeader(struct ALPHDecoder* const alph_dec,
-                          const uint8_t* const data, size_t data_size,
-                          uint8_t* const output);
+                          const uint8_t* const data, size_t data_size);
 
 // Decodes *at least* 'last_row' rows of alpha. If some of the initial rows are
 // already decoded in previous call(s), it will resume decoding from where it