[Closure Compiler GN] Enable checks_only flag for type checking compilations.

This CL adds the checks_only flag to JS Closure Compilations that are only
doing type checking to improve build time.

Before:
5869.41s user 189.50s system 3350% cpu
5861.34s user 190.37s system 3284% cpu

After:
4143.98s user 159.46s system 3020% cpu
4184.40s user 158.95s system 3073% cpu

Bug: 632206
Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: I68ebdce5a4cf98176d3f55b971e33fc62848d2d4
Reviewed-on: https://chromium-review.googlesource.com/1139840Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575918}

third_party/closure_compiler/compile_js.gni

@@ -153,6 +153,7 @@ template("js_binary") {
                              "closure_flags",
                              "externs_list",
                              "extra_deps",
+                             "checks_only",
                            ])
     args = [
       "--compiler",
@@ -202,6 +203,9 @@ template("js_binary") {
     if (!defined(closure_flags)) {
       closure_flags = default_closure_args
     }
+    if (defined(checks_only) && checks_only) {
+      args += [ "--checks-only" ]
+    }
     args += [ "--flags" ] + closure_flags
     args += [
       "--externs",
@@ -227,6 +231,7 @@ template("js_type_check") {
   if (closure_compile) {
     js_binary(target_name) {
       sources = []
+      checks_only = true
       forward_variables_from(invoker, [ "deps" ])
     }
   } else {

third_party/closure_compiler/js_binary.py

@@ -77,6 +77,8 @@ def main():
                       'Do not include leading dashes')
   parser.add_argument('-e', '--externs', nargs='*', default=[],
                       help='A list of extern files to pass to the compiler')
+  parser.add_argument('-co', '--checks-only', action='store_true',
+                      help='Only performs checks and writes an empty output')
 
   args = parser.parse_args()
   sources, externs = CrawlDepsTree(args.deps, args.sources, args.externs)
@@ -93,6 +95,10 @@ def main():
   compiler_args += args.config
   compiler_args += sources
 
+  if args.checks_only:
+    compiler_args += ['--checks-only']
+    open(args.output, 'w').close()
+
   returncode, errors = compile2.Checker().run_jar(args.compiler, compiler_args)
   if returncode != 0:
     print errors