ambient: metrics provider safer null handling

On shutdown, MetricsService may attempt to finalize logs and calls
|ProvideCurrentSessionData|. |AmbientClient| may have been destructed
by then.

BUG=1150479
TEST=on about://flags, change a flag and restart ui

Change-Id: I216b522ca1dd8c2c09a4f1350098b9c18620338c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2547331Reviewed-by: Tao Wu <wutao@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Jeffrey Young <cowmoo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#829086}

chrome/browser/metrics/ambient_mode_metrics_provider.cc

@@ -34,11 +34,15 @@ AmbientModeMetricsProvider::~AmbientModeMetricsProvider() = default;
 
 void AmbientModeMetricsProvider::ProvideCurrentSessionData(
     metrics::ChromeUserMetricsExtension* uma_proto_unused) {
-  if (!chromeos::features::IsAmbientModeEnabled() ||
-      !ash::AmbientClient::Get()->IsAmbientModeAllowed()) {
+  if (!chromeos::features::IsAmbientModeEnabled())
     return;
-  }
 
+  auto* ambient_client = ash::AmbientClient::Get();
+  if (!ambient_client || !ambient_client->IsAmbientModeAllowed())
+    return;
+
+  // |IsAmbientModeAllowed| guarantees a valid profile exists for the active
+  // user.
   PrefService* pref_service =
       ProfileManager::GetActiveUserProfile()->GetPrefs();
   DCHECK(pref_service);