gpu/khronos_glcts_support: add support for GN builds

.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 287 matching lines @@
   "//content/public/android/BUILD.gn",
   "//content/renderer/renderer.gni",
   "//content/test/BUILD.gn",
   "//content/utility/utility.gni",
   "//extensions/common/api/schemas.gni",
   "//extensions/extensions.gni",
   "//extensions/shell/app_shell.gni",
   "//extensions/shell/common/api/schemas.gni",
   "//google_apis/BUILD.gn",
   "//gpu/gles2_conform_support/BUILD.gn",
+  "//gpu/khronos_glcts_support/BUILD.gn",
   "//jingle/BUILD.gn",
   "//net/BUILD.gn",
   "//ppapi/ppapi_sources.gni",
   "//printing/BUILD.gn",
   "//remoting/host/BUILD.gn",
   "//remoting/remoting_srcs.gni",
   "//remoting/remoting_version.gni",
   "//skia/BUILD.gn",
   "//third_party/android_platform/BUILD.gn",
   "//third_party/angle/BUILD.gn",
@@ skipping 27 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",
 ]