ArcMetricsService: soft fail when unknown task_id is provided

The code assumes OnTaskCreated and OnTaskDestroyed are called in pairs
for the same task_id, soft fail if it's not the case.

Bug: 908308
Change-Id: I6d8bf5bdabbb601f53cf25e1d03edbc562613a84
Reviewed-on: https://chromium-review.googlesource.com/c/1351347Reviewed-by: Maajid <maajid@chromium.org>
Reviewed-by: Kazuhiro Inaba <kinaba@chromium.org>
Commit-Queue: Shao-Chuan Lee <shaochuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611073}

components/arc/metrics/arc_metrics_service.cc

@@ -349,7 +349,10 @@ void ArcMetricsService::OnTaskCreated(int32_t task_id,
 void ArcMetricsService::OnTaskDestroyed(int32_t task_id) {
   UpdateEngagementTime();
   auto it = std::find(task_ids_.begin(), task_ids_.end(), task_id);
-  DCHECK(it != task_ids_.end());
+  if (it == task_ids_.end()) {
+    LOG(WARNING) << "unknown task_id, background time might be undermeasured";
+    return;
+  }
   task_ids_.erase(it);
 }