Remove iOS ifdefs in src/chrome.

Review URL: https://codereview.chromium.org/1726963002

Cr-Commit-Position: refs/heads/master@{#377377}
diff --git a/chrome/browser/metrics/chrome_metrics_service_client.h b/chrome/browser/metrics/chrome_metrics_service_client.h
index 9282aba..34723b4 100644
--- a/chrome/browser/metrics/chrome_metrics_service_client.h
+++ b/chrome/browser/metrics/chrome_metrics_service_client.h
@@ -28,7 +28,7 @@
 class PrefRegistrySimple;
 class PrefService;
 
-#if !defined(OS_CHROMEOS) && !defined(OS_IOS)
+#if !defined(OS_CHROMEOS)
 class SigninStatusMetricsProvider;
 #endif