Update libwebp to version 0.5.0

DEPS

Index: DEPS
diff --git a/DEPS b/DEPS
index e40496a2bed9f8a7527286b51e970585dfd3a1a8..07d76aeb3e3255d322b0a82e44b3f76c3ceb3147 100644
--- a/DEPS
+++ b/DEPS
@@ -14,8 +14,7 @@ deps = {
   "third_party/externals/gyp"     : "https://chromium.googlesource.com/external/gyp.git@08429da7955a98ef764fafa223dd7de73f654b2d",
   "third_party/externals/harfbuzz": "https://skia.googlesource.com/third_party/harfbuzz.git@0.9.35",
   "third_party/externals/jsoncpp" : "https://chromium.googlesource.com/external/github.com/open-source-parsers/jsoncpp.git@1.0.0",
-  # Slightly ahead of v 0.4.3, to include the fix for https://bug.skia.org/4038
-  "third_party/externals/libwebp" : "https://chromium.googlesource.com/webm/libwebp.git@5ff0079ece626f122bfb8e33a5f92b5a68484176",
+  "third_party/externals/libwebp" : "https://chromium.googlesource.com/webm/libwebp.git@v0.5.0",
   "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@c4e33043fb071b6ea0a153845da625d7ed633d3d",
   # NOTE: If we update libpng, we may need to update the generated file at third_party/libpng/pnglibconf.h