[iOS CBCM] Register missing enterprise reporting pref

One of the prefs related to Enterprise reporting for iOS CBCM wasn't
being registered, and that caused a crash when CBCM was enabled. This
change correctly registers the missing pref to the local state pref
registry.

Bug: 1066495
Change-Id: I2ccc81ea8f4290cc00238618853b0c03b16de483
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2357133Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Guillaume Jenkins <gujen@google.com>
Cr-Commit-Position: refs/heads/master@{#798237}

ios/chrome/browser/prefs/browser_prefs.mm

@@ -140,6 +140,8 @@ void RegisterLocalStatePrefs(PrefRegistrySimple* registry) {
 
   registry->RegisterBooleanPref(enterprise_reporting::kCloudReportingEnabled,
                                 false);
+  registry->RegisterTimePref(enterprise_reporting::kLastUploadTimestamp,
+                             base::Time());
 }
 
 void RegisterBrowserStatePrefs(user_prefs::PrefRegistrySyncable* registry) {