[WebAuthn] Don't record IsUVPAA metric if browser process closed

ProfileListDesktopBrowserTest.Signout was flakily crashing when the
task for recording the IsUVPAA metric was executing during browser
shut down. This is a speculative fix to abort the task if the browser
process is unavailable, and also re-enable the test.

Bug: 1109768
Change-Id: I15597948850d40773c563029a8ea247f1557ef90
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2321290Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Ken Buchanan <kenrb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792041}

chrome/browser/metrics/authenticator_utility.cc

@@ -42,6 +42,10 @@ void ReportUVPlatformAuthenticatorAvailabilityWithConfig(
 
 void ReportUVPlatformAuthenticatorAvailabilityMainThreadMac() {
   DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
+  // Tests can shut down before this task is run.
+  if (!g_browser_process)
+    return;
+
   // Startup metrics are recording during PostBrowserStart() which is after
   // profile initialization. However some tests run PostBrowserStart() without
   // setting up profiles so there still needs to be a guard.

chrome/browser/profiles/profile_list_desktop_browsertest.cc

@@ -55,9 +55,9 @@ class ProfileListDesktopBrowserTest : public InProcessBrowserTest {
   DISALLOW_COPY_AND_ASSIGN(ProfileListDesktopBrowserTest);
 };
 
-#if defined(OS_WIN) || defined(OS_MACOSX)
+#if defined(OS_WIN) || (defined(OS_MACOSX) && defined(ADDRESS_SANITIZER))
 // SignOut is flaky on Windows, crbug.com/357329,
-// and Mac (crbug.com/674497, crbug.com/1109768).
+// and Mac with ASAN, crbug.com/674497.
 #define MAYBE_SignOut DISABLED_SignOut
 #elif defined(OS_CHROMEOS)
 // This test doesn't make sense for Chrome OS since it has a different