Reduce plugin_metrics_provider_ usage in MetricsService

chrome/test/base/testing_browser_process.cc

Index: chrome/test/base/testing_browser_process.cc
diff --git a/chrome/test/base/testing_browser_process.cc b/chrome/test/base/testing_browser_process.cc
index 0755d9f1f34ab44abf2abc82fd5d501f5487cb5c..196721e266dfd552ea6e3a3ff773d82115a99cd2 100644
--- a/chrome/test/base/testing_browser_process.cc
+++ b/chrome/test/base/testing_browser_process.cc
@@ -102,6 +102,10 @@ MetricsService* TestingBrowserProcess::metrics_service() {
   return NULL;
 }
 
+PluginMetricsProvider* TestingBrowserProcess::plugin_metrics_provider() {
+  return NULL;
+}
+
 rappor::RapporService* TestingBrowserProcess::rappor_service() {
   return NULL;
 }