Make fetch-gn and activate-emsdk optional in git-sync-deps

These two scripts is platform-dependent and may fail if there are no
prebuilt binaries for an unsupported platform. Making these steps
optional makes building on those platforms easier, for they can use
their own versions of toolchain (e.g. packages in a Linux distro).

Author: hack3ric

tools/git-sync-deps

@@ -19,6 +19,12 @@ Environment Variables:
 
   GIT_SYNC_DEPS_QUIET: if set to non-empty string, suppress messages.
 
+  GIT_SYNC_DEPS_NO_FETCH_GN: if set to non-empty string, bin/fetch-gn
+  will not be called.
+
+  GIT_SYNC_DEPS_NO_ACTIVATE_EMSDK: if set to non-empty string,
+  bin/activate-emsdk will not be called.
+
 Git Config:
   To disable syncing of a single repository:
       cd path/to/repository
@@ -258,18 +264,22 @@ def multithread(function, list_of_arg_lists):
 def main(argv):
   deps_file_path = os.environ.get('GIT_SYNC_DEPS_PATH', DEFAULT_DEPS_PATH)
   verbose = not bool(os.environ.get('GIT_SYNC_DEPS_QUIET', False))
+  fetch_gn = not bool(os.environ.get('GIT_SYNC_DEPS_NO_FETCH_GN', False))
+  activate_emsdk = not bool(os.environ.get('GIT_SYNC_DEPS_NO_ACTIVATE_EMSDK', False))
 
   if '--help' in argv or '-h' in argv:
     usage(deps_file_path)
     return 1
 
   git_sync_deps(deps_file_path, argv, verbose)
-  subprocess.check_call(
-      [sys.executable,
-       os.path.join(os.path.dirname(deps_file_path), 'bin', 'fetch-gn')])
-  subprocess.check_call(
-      [sys.executable,
-       os.path.join(os.path.dirname(deps_file_path), 'bin', 'activate-emsdk')])
+  if fetch_gn:
+    subprocess.check_call(
+        [sys.executable,
+        os.path.join(os.path.dirname(deps_file_path), 'bin', 'fetch-gn')])
+  if activate_emsdk:
+    subprocess.check_call(
+        [sys.executable,
+        os.path.join(os.path.dirname(deps_file_path), 'bin', 'activate-emsdk')])
   return 0