Use the upstream version of libwebp, v0.4.3.

third_party/libwebp/dsp/enc_avx2.c

Index: third_party/libwebp/dsp/enc_avx2.c
diff --git a/third_party/libwebp/dsp/enc_avx2.c b/third_party/libwebp/dsp/enc_avx2.c
new file mode 100644
index 0000000000000000000000000000000000000000..372e6169d6dcd4594c133f4a321fbfb10a5895e8
--- /dev/null
+++ b/third_party/libwebp/dsp/enc_avx2.c
@@ -0,0 +1,24 @@
+// Copyright 2014 Google Inc. All Rights Reserved.
+//
+// Use of this source code is governed by a BSD-style license
+// that can be found in the COPYING file in the root of the source
+// tree. An additional intellectual property rights grant can be found
+// in the file PATENTS. All contributing project authors may
+// be found in the AUTHORS file in the root of the source tree.
+// -----------------------------------------------------------------------------
+//
+// AVX2 version of speed-critical encoding functions.
+
+#include "./dsp.h"
+
+#if defined(WEBP_USE_AVX2)
+
+#endif  // WEBP_USE_AVX2
+
+//------------------------------------------------------------------------------
+// Entry point
+
+extern void VP8EncDspInitAVX2(void);
+
+void VP8EncDspInitAVX2(void) {
+}