Return nothing for the signed in user until the correct user can reliably be returned.

Bug: 810904
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Ibd9bae957927fa3fa78569792c58015e1b6039f0
Reviewed-on: https://chromium-review.googlesource.com/1013682Reviewed-by: Peter Lee <pkl@chromium.org>
Commit-Queue: Daniel Blakemore <danblakemore@google.com>
Cr-Commit-Position: refs/heads/master@{#551189}

ios/chrome/app/main_controller.mm

@@ -1140,8 +1140,7 @@ void MainControllerAuthenticationServiceDelegate::ClearBrowsingData(
                           ^ChromeIdentity* {
                             // TODO:(crbug.com/810904) Replace with currently
                             // signed-in user.
-                            return identityService->GetAllIdentities()
-                                .firstObject;
+                            return nil;
                           },
                           ^NSArray<ChromeIdentity*>* {
                             return identityService->GetAllIdentities();