libwebp: update snapshot to v0.3.0-rc6

third_party/libwebp/utils/quant_levels.h

Index: third_party/libwebp/utils/quant_levels.h
diff --git a/third_party/libwebp/utils/quant_levels.h b/third_party/libwebp/utils/quant_levels.h
index 89ccafe40d882daec50587b51951db86ee25f725..9f85f34bff85eb1ed2937a1be0e82347fee2320a 100644
--- a/third_party/libwebp/utils/quant_levels.h
+++ b/third_party/libwebp/utils/quant_levels.h
@@ -27,11 +27,6 @@ extern "C" {
 int QuantizeLevels(uint8_t* const data, int width, int height, int num_levels,
                    uint64_t* const sse);
 
-// Apply post-processing to input 'data' of size 'width'x'height' assuming
-// that the source was quantized to a reduced number of levels.
-// Returns false in case of error (data is NULL, invalid parameters, ...).
-int DequantizeLevels(uint8_t* const data, int width, int height);
-
 #if defined(__cplusplus) || defined(c_plusplus)
 }    // extern "C"
 #endif