[Extensions] Fix report abuse field trial crash

chrome/browser/extensions/extension_uninstall_dialog.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 "chrome/browser/extensions/extension_uninstall_dialog.h"
 
 #include "base/bind.h"
 #include "base/logging.h"
 #include "base/message_loop/message_loop.h"
 #include "base/metrics/field_trial.h"
@@ skipping 114 matching lines @@
         base::UTF8ToUTF16(extension_->name()));
   }
   return l10n_util::GetStringFUTF8(IDS_EXTENSION_UNINSTALL_PROMPT_HEADING,
                                    base::UTF8ToUTF16(extension_->name()));
 }
 
 bool ExtensionUninstallDialog::ShouldShowReportAbuseCheckbox() const {
   static const char kExperimentName[] = "ExtensionUninstall.ReportAbuse";
   static const char kDefaultGroupName[] = "Default";
   static const char kShowCheckboxGroup[] = "ShowCheckbox";
-  // TODO(devlin): Turn on this field trial. See crbug.com/441377.
-  scoped_refptr<base::FieldTrial> trial(
-      base::FieldTrialList::FactoryGetFieldTrial(
-          kExperimentName,
-          100,  // Total probability.
-          kDefaultGroupName,
-          2015, 7, 31,  // End date.
-          base::FieldTrial::ONE_TIME_RANDOMIZED,
-          nullptr));
-  int experiment_group = trial->AppendGroup(kShowCheckboxGroup, 0);
-  return base::FieldTrialList::FindValue(kExperimentName) == experiment_group;
+
+  // Important: Only initialize the trial once.
+  if (!base::FieldTrialList::Find(kExperimentName)) {
+    // TODO(devlin): Turn on this field trial. See crbug.com/441377.

[Alexei Svitkine (slow), 2015/03/31 20:50:53]

The client-side API to create a field trial, like you're doing here is
discouraged.

It's best to control the trial from the server-side, in which case you don't
need this whole block.

Happy to answer any question on how to do this.

+    scoped_refptr<base::FieldTrial> trial(
+        base::FieldTrialList::FactoryGetFieldTrial(
+            kExperimentName,
+            100,  // Total probability.
+            kDefaultGroupName,
+            2015, 7, 31,  // End date.
+            base::FieldTrial::ONE_TIME_RANDOMIZED,
+            nullptr));
+    trial->AppendGroup(kShowCheckboxGroup, 0);
+  }
+
+  return base::FieldTrialList::FindFullName(kExperimentName) ==
+      kShowCheckboxGroup;
 }
 
 void ExtensionUninstallDialog::HandleReportAbuse() {
   chrome::NavigateParams params(
       profile_,
       extension_urls::GetWebstoreReportAbuseUrl(extension_->id()),
       ui::PAGE_TRANSITION_LINK);
   params.disposition = NEW_FOREGROUND_TAB;
   chrome::Navigate(&params);
 }
 
 }  // namespace extensions