Use the upstream version of libwebp, v0.4.3.

DEPS

Index: DEPS
diff --git a/DEPS b/DEPS
index 318cf4e3c88c928cdfd68bd0ac4c26c56c679919..bb44306e26496f37ddce7dff3e30f2f21ed6cffd 100644
--- a/DEPS
+++ b/DEPS
@@ -14,7 +14,7 @@ deps = {
   "third_party/externals/harfbuzz": "https://skia.googlesource.com/third_party/harfbuzz.git@0.9.35",
   "third_party/externals/jsoncpp" : "https://chromium.googlesource.com/external/jsoncpp/jsoncpp.git@1afff032c83e26ddf7f2776e8b43de5ad666c1fa",
   "third_party/externals/libjpeg" : "https://chromium.googlesource.com/chromium/deps/libjpeg_turbo.git@034e9a9747e0983bc19808ea70e469bc8342081f",
-  "third_party/externals/libwebp" : "https://chromium.googlesource.com/webm/libwebp.git@3fe91635df8734b23f3c1b9d1f0c4fa8cfaf4e39",
+  "third_party/externals/libwebp" : "https://chromium.googlesource.com/webm/libwebp.git@v0.4.3",
   "third_party/externals/nanomsg" : "https://skia.googlesource.com/third_party/nanomsg.git@0.4-beta",
   "third_party/externals/zlib"    : "https://chromium.googlesource.com/chromium/src/third_party/zlib@4ba7cdd0e7bf49d671645264f839838fc56e1492",
   # NOTE: If we update libpng, we may need to update the generated file at third_party/libpng/pnglibconf.h