Update libwebp to 0.6.0

DEPS

 use_relative_paths = True
 
 # Dependencies on outside packages.
 #
 deps = {
   "buildtools":  "https://chromium.googlesource.com/chromium/buildtools.git@55ad626b08ef971fd82a62b7abb325359542952b",
 
   "common": "https://skia.googlesource.com/common.git@9737551d7a52c3db3262db5856e6bcd62c462b92",
 
   # There is some duplication here that might be worth cleaning up:
   #   - can use use our existing t_p/e/libjpeg instead of pulling it for Android?
 
   "third_party/externals/angle2"  : "https://chromium.googlesource.com/angle/angle.git@57f17473791703ac82add77c3d77d13d8e2dfbc4",
   "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",
   "third_party/externals/piex"    : "https://android.googlesource.com/platform/external/piex.git@8f540f64b6c170a16fb7e6e52d61819705c1522a",
 
   "third_party/externals/libjpeg-turbo"             : "https://skia.googlesource.com/third_party/libjpeg-turbo.git@6de58e0d28014caf2fc1370145f22fd6d65f63e3",
   # libjpeg-turbo depends on yasm to compile .asm files
   "third_party/externals/yasm/source/patched-yasm/" : "https://chromium.googlesource.com/chromium/deps/yasm/patched-yasm.git@4671120cd8558ce62ee8672ebf3eb6f5216f909b",
   "third_party/externals/yasm/binaries"             : "https://chromium.googlesource.com/chromium/deps/yasm/binaries.git@52f9b3f4b0aa06da24ef8b123058bb61ee468881",
 
@@ skipping 19 matching lines @@
 }
 
 recursedeps = [ "common" ]
 
 hooks = []
 
 import os
 import sys
 if os.path.exists('bin/fetch-gn'):
   hooks.append({ 'action': [sys.executable, 'bin/fetch-gn'] })