Make ScopedProfileKeepAlive play nice in unit tests

In unit-tests, g_browser_process->profile_manager() can return null.
When the DestroyProfileOnBrowserClose flag is enabled (via
ScopedFeatureList, or fieldtrial_testing_config.json),
ScopedProfileKeepAlive causes a crash.

Add a guard in ScopedProfileKeepAlive(), so it's a no-op if there is no
ProfileManager.

Bug: 88586
Change-Id: Ife9f198da3d7dc69f8cf59bed80aca9c57517a6d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2614859
Auto-Submit: Nicolas Ouellet-Payeur <nicolaso@chromium.org>
Commit-Queue: Mihai Sardarescu <msarda@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841088}

chrome/browser/profiles/scoped_profile_keep_alive.cc

@@ -12,7 +12,10 @@ ScopedProfileKeepAlive::ScopedProfileKeepAlive(const Profile* profile,
                                                ProfileKeepAliveOrigin origin)
     : profile_(profile), origin_(origin) {
   DCHECK(profile_);
-  g_browser_process->profile_manager()->AddKeepAlive(profile_, origin_);
+  // |profile_manager| can be nullptr in tests.
+  auto* profile_manager = g_browser_process->profile_manager();
+  if (profile_manager)
+    profile_manager->AddKeepAlive(profile_, origin_);
 }
 
 ScopedProfileKeepAlive::~ScopedProfileKeepAlive() {