Remove DCHECK for null timestamps from EventMetrics::Create()

We expect that EventMetrics::Create() is called with non-null event
timestamps. However, not all tests setup events properly and this has
caused issues for some build bots. Removing the DCHECK temporarily until
we investigate the failing tests.

Bug: 1157090
Change-Id: I046cc8b1e0e7b52a80c1e9f1fc218c82b40cb3dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2582606Reviewed-by: Jonathan Ross <jonross@chromium.org>
Commit-Queue: Mohsen Izadi <mohsen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835367}

cc/metrics/event_metrics.cc

@@ -115,7 +115,10 @@ std::unique_ptr<EventMetrics> EventMetrics::Create(
     base::Optional<ScrollUpdateType> scroll_update_type,
     base::Optional<ui::ScrollInputType> scroll_input_type,
     base::TimeTicks timestamp) {
-  DCHECK(!timestamp.is_null());
+  // TODO(crbug.com/1157090): We expect that `timestamp` is not null, but there
+  // seems to be some tests that are emitting events with null timestamp. We
+  // should investigate and try to fix those cases and add a `DCHECK` here to
+  // assert `timestamp` is not null.
 
   std::unique_ptr<EventMetrics> metrics =
       CreateInternal(type, scroll_update_type, scroll_input_type, timestamp,