Add a private API method metricsPrivate.getFieldTrial(), which returns

chrome/browser/extensions/api/metrics_private/metrics_apitest.cc

Index: chrome/browser/extensions/api/metrics_private/metrics_apitest.cc
diff --git a/chrome/browser/extensions/api/metrics_private/metrics_apitest.cc b/chrome/browser/extensions/api/metrics_private/metrics_apitest.cc
index 655670fc16baa0a9985d00ca6ec3ab171a9909c1..a4a36cac70bf2f8ba705ea9090f4361091bfcb8a 100644
--- a/chrome/browser/extensions/api/metrics_private/metrics_apitest.cc
+++ b/chrome/browser/extensions/api/metrics_private/metrics_apitest.cc
@@ -4,6 +4,7 @@
 
 #include <map>
 
+#include "base/metrics/field_trial.h"
 #include "base/metrics/histogram.h"
 #include "base/metrics/statistics_recorder.h"
 #include "chrome/browser/extensions/extension_apitest.h"
@@ -126,6 +127,8 @@ void ValidateHistograms(const RecordedHistogram* recorded,
 IN_PROC_BROWSER_TEST_F(ExtensionApiTest, Metrics) {
   UserActionObserver observer;
 
+  base::FieldTrialList::CreateTrialsFromString("apitestfieldtrial2/group1/");
+
   ASSERT_TRUE(RunComponentExtensionTest("metrics")) << message_;
 
   observer.ValidateUserActions(g_user_actions, arraysize(g_user_actions));