libwebp: update to v0.5.1

third_party/libwebp/dec/alphai.h

Index: third_party/libwebp/dec/alphai.h
diff --git a/third_party/libwebp/dec/alphai.h b/third_party/libwebp/dec/alphai.h
index 5fa230ca8258e67cb200e7b4a8810b5a830fe497..69dd7c0f5d610db7ba218402b785d180f2aed2d3 100644
--- a/third_party/libwebp/dec/alphai.h
+++ b/third_party/libwebp/dec/alphai.h
@@ -32,19 +32,18 @@ struct ALPHDecoder {
   int pre_processing_;
   struct VP8LDecoder* vp8l_dec_;
   VP8Io io_;
-  int use_8b_decode;  // Although alpha channel requires only 1 byte per
-                      // pixel, sometimes VP8LDecoder may need to allocate
-                      // 4 bytes per pixel internally during decode.
+  int use_8b_decode_;  // Although alpha channel requires only 1 byte per
+                       // pixel, sometimes VP8LDecoder may need to allocate
+                       // 4 bytes per pixel internally during decode.
+  uint8_t* output_;
+  const uint8_t* prev_line_;   // last output row (or NULL)
 };
 
 //------------------------------------------------------------------------------
 // internal functions. Not public.
 
-// Allocates a new alpha decoder instance.
-ALPHDecoder* ALPHNew(void);
-
-// Clears and deallocates an alpha decoder instance.
-void ALPHDelete(ALPHDecoder* const dec);
+// Deallocate memory associated to dec->alpha_plane_ decoding
+void WebPDeallocateAlphaMemory(VP8Decoder* const dec);
 
 //------------------------------------------------------------------------------