Add support for the GN configuration language (#3368)

* Add samples and definition for GN build files

* Add grammar to provide GN syntax highlighting

* Fix failing tests

* Add Python extensions for GYP includes and .gclient configs

samples/GN/filenames/.gn

@@ -0,0 +1,25 @@
+# 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.
+
+import("//build/dotfile_settings.gni")
+
+# 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.
+secondary_source = "//build/secondary/"
+
+# These are the targets to check headers for by default. The files in targets
+# matching these patterns (see "gn help label_pattern" for format) will have
+# their includes checked for proper dependencies when you run either
+# "gn check" or "gn gen --check".
+check_targets = []
+
+# 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
+# discouraged except for gypi_to_gn calls.
+exec_script_whitelist =
+    build_dotfile_settings.exec_script_whitelist + [ "//test/test262/BUILD.gn" ]