Make --force-fieldtrials not activate them in the browser process.

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

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #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"
@@ skipping 109 matching lines @@
     }
     EXPECT_LT(j, histograms.size());
   }
 }
 
 }  // anonymous namespace
 
 IN_PROC_BROWSER_TEST_F(ExtensionApiTest, Metrics) {
   UserActionObserver observer;
 
-  base::FieldTrialList::CreateTrialsFromString("apitestfieldtrial2/group1/");
+  base::FieldTrialList::CreateFieldTrial("apitestfieldtrial2", "group1");
 
   ASSERT_TRUE(RunComponentExtensionTest("metrics")) << message_;
 
   observer.ValidateUserActions(g_user_actions, arraysize(g_user_actions));
   ValidateHistograms(g_histograms, arraysize(g_histograms));
 }