OS Settings: Only fetch device name if the flag is enabled.

This change fixes a crash where the device name is fetched from the
DeviceNameStore, which is not initiatlized when the flag is off.

R=khorimoto@chromium.org

Bug: 1151252
Test: none
Change-Id: I314b6bd27f3bfe1ff7bce9ebf9ebd7c28edf8188
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2552671Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: James Hawkins <jhawkins@chromium.org>
Cr-Commit-Position: refs/heads/master@{#829850}

chrome/browser/resources/settings/chromeos/os_about_page/detailed_build_info.js

@@ -75,7 +75,10 @@ Polymer({
     });
 
     this.updateChannelInfo_();
-    this.updateDeviceName_();
+
+    if (this.isHostnameSettingEnabled_) {
+      this.updateDeviceName_();
+    }
   },
 
   /**