[Extensions] Fix report abuse field trial crash

Field trials can only be initialized once, so the FactoryGet() method cannot
be called each time to check the value of the trial.

BUG=470278

Review URL: https://codereview.chromium.org/1023673011

Cr-Commit-Position: refs/heads/master@{#322212}

chrome/browser/extensions/extension_uninstall_dialog.cc

@@ -132,6 +132,9 @@ bool ExtensionUninstallDialog::ShouldShowReportAbuseCheckbox() const {
   static const char kExperimentName[] = "ExtensionUninstall.ReportAbuse";
   static const char kDefaultGroupName[] = "Default";
   static const char kShowCheckboxGroup[] = "ShowCheckbox";
+
+  // Important: Only initialize the trial once.
+  if (!base::FieldTrialList::Find(kExperimentName)) {
     // TODO(devlin): Turn on this field trial. See crbug.com/441377.
     scoped_refptr<base::FieldTrial> trial(
         base::FieldTrialList::FactoryGetFieldTrial(
@@ -141,8 +144,11 @@ bool ExtensionUninstallDialog::ShouldShowReportAbuseCheckbox() const {
             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;
+    trial->AppendGroup(kShowCheckboxGroup, 0);
+  }
+
+  return base::FieldTrialList::FindFullName(kExperimentName) ==
+      kShowCheckboxGroup;
 }
 
 void ExtensionUninstallDialog::HandleReportAbuse() {