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

runtime/third_party/double-conversion/src/BUILD.gn

Index: runtime/third_party/double-conversion/src/BUILD.gn
diff --git a/runtime/third_party/double-conversion/src/BUILD.gn b/runtime/third_party/double-conversion/src/BUILD.gn
index 5aaa53b25f212f4c012ec11a79b8ac6beeb0f821..788862338d05620f52fdc574c40d44ff542809d4 100644
--- a/runtime/third_party/double-conversion/src/BUILD.gn
+++ b/runtime/third_party/double-conversion/src/BUILD.gn
@@ -2,14 +2,13 @@
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
-
 static_library("libdouble_conversion") {
-  configs += [ "../../..:dart_config", ]
+  configs += [ "../../..:dart_config" ]
   sources = [
-    "bignum.cc",
-    "bignum.h",
     "bignum-dtoa.cc",
     "bignum-dtoa.h",
+    "bignum.cc",
+    "bignum.h",
     "cached-powers.cc",
     "cached-powers.h",
     "diy-fp.cc",
@@ -25,8 +24,5 @@ static_library("libdouble_conversion") {
     "strtod.h",
     "utils.h",
   ]
-  include_dirs = [
-    ".",
-  ]
+  include_dirs = [ "." ]
 }
-