Add a startup benchmark and rudimentary scripts for running benchmarks.

benchmarks/BUILD.gn

Index: benchmarks/BUILD.gn
diff --git a/mojo/public/js/BUILD.gn b/benchmarks/BUILD.gn
similarity index 67%
copy from mojo/public/js/BUILD.gn
copy to benchmarks/BUILD.gn
index 4b86e6eb9a562f6c158fd557c8d67ee633dbc722..e1de13aa76441597b206c2f3379c785355d7544f 100644
--- a/mojo/public/js/BUILD.gn
+++ b/benchmarks/BUILD.gn
@@ -2,9 +2,10 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-source_set("js") {
-  sources = [
-    "constants.cc",
-    "constants.h",
+group("benchmarks") {
+  testonly = true
+
+  deps = [
+    "//benchmarks/startup",
   ]
 }