[Profile picker] Refactor startup browser tests

This CL refactors tests for startup browser creator so that (more of)
the real production startup code is exercised. This change is needed
by CL 2547040 and splits off refactoring and behavioral change.

Bug: 1150330
Change-Id: Ia81dc71ea092ad12001d355c93557f77ee5d7c4e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2563548Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Tommy Martino <tmartino@chromium.org>
Commit-Queue: Robert Kaplow <rkaplow@chromium.org>
Auto-Submit: Jan Krcal <jkrcal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#832348}

chrome/browser/metrics/authenticator_utility.cc

@@ -56,7 +56,11 @@ void ReportUVPlatformAuthenticatorAvailabilityMainThreadMac() {
   }
   Profile* profile = profile_manager->GetProfileByPath(
       profile_manager->GetLastUsedProfileDir(profile_manager->user_data_dir()));
-  DCHECK(profile);
+  // Some tests have profiles but do not load the last profile before
+  // PostBrowserStart().
+  if (!profile) {
+    return;
+  }
 
   // Return to a low-priority thread for the actual check.
   base::ThreadPool::PostTask(