Include //third_party/WebKit/Source/* in default gn check targets.

.gn

 # This file is used by the GN meta build system to find the root of the source
 # tree and to set startup options. For documentation on the values set in this
 # file, run "gn help dotfile" at the command line.
 
 # The location of the build configuration file.
 buildconfig = "//build/config/BUILDCONFIG.gn"
 
 # The secondary source root is a parallel directory tree where
 # GN build files are placed when they can not be placed directly
 # in the source tree, e.g. for third party source trees.
@@ skipping 215 matching lines @@
   #"//rlz/*",  # Needs checking on Windows.
   #"//sandbox/*",  # Medium-hard.
   "//sdch/*",
   "//skia/*",
   "//sql/*",
   "//storage/*",
   "//sync/*",
   "//testing/*",
 
   #"//third_party/*",  # May not ever want this.
+  "//third_party/WebKit/Source/*",
   "//tools/*",
 
   #"//ui/*",  # Just a few problems.
   "//url/*",
   "//v8/*",
   "//win8/*",
 ]
 
 # These are the list of GN files that run exec_script. This whitelist exists
 # to force additional review for new uses of exec_script, which is strongly
@@ skipping 99 matching lines @@
   "//tools/gn/format_test_data/055.golden",
   "//tools/gn/function_exec_script.cc",
   "//tools/gn/gn.gyp",
   "//tools/gn/input_conversion.cc",
   "//tools/gn/misc/emacs/gn.el",
   "//tools/gn/misc/vim/syntax/gn.vim",
   "//tools/gn/setup.cc",
   "//ui/accessibility/BUILD.gn",
   "//ui/views/BUILD.gn",
 ]