Add //testing/perf and fix up output file location in yasm_assemble.gni.

testing/perf/BUILD.gn

Index: testing/perf/BUILD.gn
diff --git a/content/public/plugin/BUILD.gn b/testing/perf/BUILD.gn
similarity index 69%
copy from content/public/plugin/BUILD.gn
copy to testing/perf/BUILD.gn
index 059aea60ebde90b69dc745b093d184dd8a793019..d158f1988a2c7b8fc6481948428d91f40eadeee8 100644
--- a/content/public/plugin/BUILD.gn
+++ b/testing/perf/BUILD.gn
@@ -2,8 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-source_set("plugin") {
-  sources = [
-    "content_plugin_client.h",
-  ]
+source_set("perf") {
+  sources = [ "perf_test.cc" ]
+  deps = [ "//base" ]
 }