[ios] Only call RecordMetricsReportingDefaultState once.

Even behind a dispatch_once, WelcomeToChromeViewController can be
displayed multiple times -- such as if the app is quit before accepting
the tos, or by test when using experiment settings.

Bug: 741003
Change-Id: I54c7ff333ba8c8b48ecb55dfc7fab649ed667243
Reviewed-on: https://chromium-review.googlesource.com/c/1489099
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Peter Lee <pkl@chromium.org>
Auto-Submit: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636334}

ios/chrome/browser/ui/first_run/welcome_to_chrome_view_controller.mm

@@ -88,6 +88,14 @@ const BOOL kDefaultStatsCheckboxValue = YES;
   // Record metrics reporting as opt-in/opt-out only once.
   static dispatch_once_t once;
   dispatch_once(&once, ^{
+    // Don't call RecordMetricsReportingDefaultState twice.  This can happen
+    // if the app is quit before accepting the TOS, or via experiment settings.
+    if (metrics::GetMetricsReportingDefaultState(
+            GetApplicationContext()->GetLocalState()) !=
+        metrics::EnableMetricsDefault::DEFAULT_UNKNOWN) {
+      return;
+    }
+
     metrics::RecordMetricsReportingDefaultState(
         GetApplicationContext()->GetLocalState(),
         kDefaultStatsCheckboxValue ? metrics::EnableMetricsDefault::OPT_OUT