LatencyInfo trace_id_ no longer dependent on sequence_number.

content/browser/renderer_host/input/render_widget_host_latency_tracker.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/browser/renderer_host/input/render_widget_host_latency_tracker.h"
 
 #include <stddef.h>
 
 #include "base/logging.h"
 #include "base/metrics/histogram_functions.h"
 #include "base/metrics/histogram_macros.h"
 #include "build/build_config.h"
 #include "components/rappor/public/rappor_utils.h"
 #include "components/ukm/public/ukm_entry_builder.h"
 #include "content/browser/renderer_host/render_widget_host_delegate.h"
+#include "content/public/browser/browser_thread.h"
 #include "content/public/browser/content_browser_client.h"
 #include "content/public/common/content_client.h"
 #include "ui/events/blink/web_input_event_traits.h"
 #include "ui/latency/latency_histogram_macros.h"
 
 using blink::WebGestureEvent;
 using blink::WebInputEvent;
 using blink::WebMouseEvent;
 using blink::WebMouseWheelEvent;
 using blink::WebTouchEvent;
@@ skipping 160 matching lines @@
           "Event.Latency.Browser." + input_modality + "Acked", rwh_component,
           acked_component);
     }
   }
 }
 
 void RenderWidgetHostLatencyTracker::OnInputEvent(
     const blink::WebInputEvent& event,
     LatencyInfo* latency) {
   DCHECK(latency);
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+
+  static uint64_t global_trace_id = 0;
+  latency->set_trace_id(++global_trace_id);
 
   if (event.GetType() == WebInputEvent::kTouchStart) {
     const WebTouchEvent& touch_event =
         *static_cast<const WebTouchEvent*>(&event);
     DCHECK(touch_event.touches_length >= 1);
     active_multi_finger_gesture_ = touch_event.touches_length != 1;
   }
 
   if (latency->source_event_type() == ui::KEY_PRESS) {
     DCHECK(event.GetType() == WebInputEvent::kChar ||
@@ skipping 144 matching lines @@
   if (ukm_source_id == -1 || !ukm_recorder)
     return;
 
   std::unique_ptr<ukm::UkmEntryBuilder> builder =
       ukm_recorder->GetEntryBuilder(ukm_source_id, event_name.c_str());
   builder->AddMetric(metric_name.c_str(), (end_component.last_event_time -
                                            start_component.first_event_time)
                                               .InMicroseconds());
 }
 }  // namespace content