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

runtime/observatory/BUILD.gn

Index: runtime/observatory/BUILD.gn
diff --git a/runtime/observatory/BUILD.gn b/runtime/observatory/BUILD.gn
index fc7af6a8b788af9aa49d1abbbc3ef1e246cee6d3..5058eed80c26854096b03f541a929d7435b98d50 100644
--- a/runtime/observatory/BUILD.gn
+++ b/runtime/observatory/BUILD.gn
@@ -16,15 +16,12 @@ declare_args() {
 # 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 = [
     "$root_gen_dir/observatory_copy/{{source_root_relative_dir}}/{{source_file_part}}",
   ]
@@ -51,8 +48,7 @@ action("write_observatory_pubspec_yaml") {
     "--command",
     "rewrite",
     rebase_path("../observatory/pubspec.yaml"),
-    rebase_path(
-        "$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/"),
   ]
@@ -94,10 +90,9 @@ action("copy_observatory_deps") {
 
 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",
@@ -184,10 +179,14 @@ template("observatory_archive") {
       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" ]