Format GN files and add a presubmit check that GN is properly formatted

runtime/observatory/BUILD.gn

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Currently paths here are hard coded for convenience in building Mojo/Flutter.
 declare_args() {
   # Specify the path to a host compatible version of the Dart SDK.
   # This SDK is used to compile the Observatory frontend sources.
   dart_host_sdk = rebase_path("//third_party/dart-sdk/dart-sdk")
 
   # Specify the path to a host compatible version of pub.
   # This is used to compile the Observatory frontend sources.
   dart_host_pub_exe = rebase_path("$dart_host_sdk/bin/pub")
 }
 
 # Helper build rules for packaging the Dart observatory resources.
 observatory_sources_gypi =
     exec_script("../../tools/gypi_to_gn.py",
-                [ rebase_path(
-                    "../observatory/observatory_sources.gypi") ],
+                [ rebase_path("../observatory/observatory_sources.gypi") ],
                 "scope",
                 [ "../observatory/observatory_sources.gypi" ])
 
 copy("copy_observatory") {
-  sources = rebase_path(observatory_sources_gypi.sources,
-                        "",
-                        ".")
+  sources = rebase_path(observatory_sources_gypi.sources, "", ".")
   outputs = [
     "$root_gen_dir/observatory_copy/{{source_root_relative_dir}}/{{source_file_part}}",
   ]
 }
 
 action("write_observatory_pubspec_yaml") {
   deps = [
     ":copy_observatory",
   ]
 
   script = "../../tools/observatory_tool.py"
 
   inputs = [
     rebase_path("pubspec.yaml"),
   ]
 
   current_dir = rebase_path(".", "//")
   args = [
     "--silent=True",
     "--pub-executable",
     dart_host_pub_exe,
     "--directory",
     rebase_path("$root_gen_dir/observatory_copy/$current_dir/"),
     "--command",
     "rewrite",
     rebase_path("../observatory/pubspec.yaml"),
-    rebase_path(
-        "$root_gen_dir/observatory_copy/$current_dir/pubspec.yaml"),
+    rebase_path("$root_gen_dir/observatory_copy/$current_dir/pubspec.yaml"),
     "../../third_party/",
     rebase_path("../../third_party/"),
   ]
 
   outputs = [
     "$root_gen_dir/observatory_copy/$current_dir/pubspec.yaml",
   ]
 }
 
 action("copy_observatory_deps") {
@@ skipping 21 matching lines @@
     "get",
   ]
 
   outputs = [
     "$root_gen_dir/observatory_copy/$current_dir/packages.stamp",
   ]
 }
 
 action("pub_build_observatory") {
   current_dir = rebase_path(".", "//")
-  sources =
-      rebase_path(observatory_sources_gypi.sources,
-                  "",
-                  "$root_gen_dir/observatory_copy/$current_dir")
+  sources = rebase_path(observatory_sources_gypi.sources,
+                        "",
+                        "$root_gen_dir/observatory_copy/$current_dir")
 
   deps = [
     ":copy_observatory",
     ":copy_observatory_deps",
   ]
 
   script = "../../tools/observatory_tool.py"
 
   inputs = [
     script,
@@ skipping 66 matching lines @@
     ]
 
     inner_namespace = invoker.inner_namespace
     outer_namespace = invoker.outer_namespace
     output_name = target_name
     args = [
       "--output",
       rebase_path("$root_gen_dir/observatory/${output_name}.cc"),
       "--tar_output",
       rebase_path("$root_gen_dir/observatory/${output_name}.tar"),
-      "--outer_namespace", outer_namespace,
-      "--inner_namespace", inner_namespace,
-      "--name", "observatory_assets_archive",
-      "--client_root", rebase_path("$root_out_dir/observatory/deployed/web/"),
+      "--outer_namespace",
+      outer_namespace,
+      "--inner_namespace",
+      inner_namespace,
+      "--name",
+      "observatory_assets_archive",
+      "--client_root",
+      rebase_path("$root_out_dir/observatory/deployed/web/"),
     ]
     if (enable_compression) {
       args += [ "--compress" ]
     }
 
     outputs = [
       "$root_gen_dir/observatory/${output_name}.cc",
       "$root_gen_dir/observatory/${output_name}.tar",
     ]
   }
@@ skipping 22 matching lines @@
 
 source_set("standalone_observatory_archive") {
   deps = [
     ":standalone_archive_observatory",
   ]
 
   sources = [
     rebase_path("$root_gen_dir/observatory/standalone_archive_observatory.cc"),
   ]
 }