Make ScrollUpdate LatencyInfo's sequence number less likely to have conflicts

cc/trees/latency_info_swap_promise_monitor.cc

 // Copyright 2013 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 "cc/trees/latency_info_swap_promise_monitor.h"
 
 #include "base/threading/platform_thread.h"
 #include "cc/output/latency_info_swap_promise.h"
 #include "cc/trees/layer_tree_host.h"
 #include "cc/trees/layer_tree_host_impl.h"
@@ skipping 53 matching lines @@
 }
 
 void LatencyInfoSwapPromiseMonitor::OnForwardScrollUpdateToMainThreadOnImpl() {
   if (AddForwardingScrollUpdateToMainComponent(latency_)) {
     int64 new_sequence_number = 0;
     for (ui::LatencyInfo::LatencyMap::const_iterator it =
              latency_->latency_components.begin();
          it != latency_->latency_components.end(); ++it) {
       if (it->first.first == ui::INPUT_EVENT_LATENCY_BEGIN_RWH_COMPONENT) {
         new_sequence_number =
-            (static_cast<int64>(base::PlatformThread::CurrentId()) << 32) |
+            ((static_cast<int64>(base::PlatformThread::CurrentId()) << 32) ^
+             (std::abs(reinterpret_cast<int64>(this)) << 32)) |

[Sami, 2015/04/16 10:55:44]

Could you replace the abs() with a cast to uint64 before shifting?

[Yufeng Shen (Slow to review), 2015/04/16 15:19:54]

Done.

             (it->second.sequence_number & 0xffffffff);
-        DCHECK(new_sequence_number != it->second.sequence_number);
+        if (new_sequence_number == it->second.sequence_number)
+          return;
         break;
       }
     }
     if (!new_sequence_number)
       return;
     ui::LatencyInfo new_latency;
     new_latency.TraceEventType("ScrollUpdate");
     new_latency.AddLatencyNumber(
         ui::INPUT_EVENT_LATENCY_BEGIN_SCROLL_UPDATE_MAIN_COMPONENT, 0,
         new_sequence_number);
     new_latency.CopyLatencyFrom(
         *latency_,
         ui::INPUT_EVENT_LATENCY_FORWARD_SCROLL_UPDATE_TO_MAIN_COMPONENT);
     scoped_ptr<SwapPromise> swap_promise(
         new LatencyInfoSwapPromise(new_latency));
     layer_tree_host_impl_->QueueSwapPromiseForMainThreadScrollUpdate(
         swap_promise.Pass());
   }
 }
 
 }  // namespace cc