Roll libwebp to v0.4.2 (latest stable) to fix annoying build warning.

DEPS

Index: DEPS
diff --git a/DEPS b/DEPS
index 02b9182ef17c9e7719ecca516f6b082f9916d412..41feed91100c38c50bb8f94658f08fff8e7c5f82 100644
--- a/DEPS
+++ b/DEPS
@@ -15,7 +15,7 @@ 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@82ce8a6d4ebe12a177c0c3597192f2b4f09e81c3",
-  "third_party/externals/libwebp" : "https://chromium.googlesource.com/webm/libwebp.git@3fe91635df8734b23f3c1b9d1f0c4fa8cfaf4e39",
+  "third_party/externals/libwebp" : "https://chromium.googlesource.com/webm/libwebp.git@v0.4.2",
   "third_party/externals/nanomsg": "https://skia.googlesource.com/third_party/nanomsg.git@0.4-beta",
 
   "platform_tools/android/third_party/externals/expat" : "https://android.googlesource.com/platform/external/expat.git@android-4.2.2_r1.2",