Update libwebp to 0.6.0

DEPS

Index: DEPS
diff --git a/DEPS b/DEPS
index 4617ca70f15aee729bda4d5cbb3d644cd01081c8..2fe61a84fff6d8ad70af421b27ffbbdcf4b9f0c4 100644
--- a/DEPS
+++ b/DEPS
@@ -14,7 +14,7 @@ deps = {
   "third_party/externals/freetype": "https://skia.googlesource.com/third_party/freetype2.git@08fd250e1af0aa16d18012d39462e6ca9bbc6e90",
   "third_party/externals/harfbuzz": "https://skia.googlesource.com/third_party/harfbuzz.git@1.4.2",
   "third_party/externals/jsoncpp" : "https://chromium.googlesource.com/external/github.com/open-source-parsers/jsoncpp.git@1.0.0",
-  "third_party/externals/libwebp" : "https://chromium.googlesource.com/webm/libwebp.git@v0.5.2-rc2",
+  "third_party/externals/libwebp" : "https://chromium.googlesource.com/webm/libwebp.git@v0.6.0",
   "third_party/externals/zlib"    : "https://chromium.googlesource.com/chromium/src/third_party/zlib@4576304a4b9835aa8646c9735b079e1d96858633",
 
   "third_party/externals/dng_sdk" : "https://android.googlesource.com/platform/external/dng_sdk.git@96443b262250c390b0caefbf3eed8463ba35ecae",