Remove incorrect DCHECK from WindowPerformance::RegisterEventTiming

Clusterfuzz tells us that GetFrame() could be null when
WindowPerformance::RegisterEventTiming is called, so this CL removes
the DCHECK and bails out early instead.

Bug: 861627
Change-Id: Ic109e8153a80832324d9c3cf27e1bed957d2b1c2
Reviewed-on: https://chromium-review.googlesource.com/1131226
Reviewed-by: Steve Kobes <skobes@chromium.org>
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573769}

third_party/blink/renderer/core/timing/window_performance.cc

@@ -327,12 +327,13 @@ void WindowPerformance::RegisterEventTiming(String event_type,
   DCHECK(!processing_start.is_null());
   DCHECK(!processing_end.is_null());
   DCHECK_GE(processing_end, processing_start);
+  if (!GetFrame())
+    return;
+
   PerformanceEventTiming* entry = PerformanceEventTiming::Create(
       event_type, MonotonicTimeToDOMHighResTimeStamp(start_time),
       MonotonicTimeToDOMHighResTimeStamp(processing_start),
       MonotonicTimeToDOMHighResTimeStamp(processing_end), cancelable);
-
-  DCHECK(GetFrame());
   event_timings_.push_back(entry);
   WebLayerTreeView* layerTreeView =
       GetFrame()->GetChromeClient().GetWebLayerTreeView(GetFrame());