GN: Add an assert that sysroot exists

.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 88 matching lines @@
   "//build/config/android/config.gni",
   "//build/config/android/internal_rules.gni",
   "//build/config/android/rules.gni",
   "//build/config/compiler/BUILD.gn",
   "//build/config/gcc/gcc_version.gni",
   "//build/config/ios/ios_sdk.gni",
   "//build/config/linux/BUILD.gn",
   "//build/config/linux/pkg_config.gni",
   "//build/config/mac/mac_sdk.gni",
   "//build/config/posix/BUILD.gn",
+  "//build/config/sysroot.gni",
   "//build/config/win/BUILD.gn",
   "//build/config/win/visual_studio_version.gni",
   "//build/gn_helpers.py",
   "//build/gypi_to_gn.py",
   "//build/linux/BUILD.gn",
   "//build/toolchain/gcc_toolchain.gni",
   "//build/toolchain/mac/BUILD.gn",
   "//build/toolchain/nacl/BUILD.gn",
   "//build/toolchain/win/BUILD.gn",
   "//build/util/version.gni",
@@ skipping 75 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",
 ]