Give MainThreadEventQueue a mutable WebInputEvent.

content/common/input/event_with_latency_info.cc

Index: content/common/input/event_with_latency_info.cc
diff --git a/content/common/input/event_with_latency_info.cc b/content/common/input/event_with_latency_info.cc
index 49359a129e8f64321e317396c78310efb2b36109..0fc592bee8471de7d8b8d977227504cc6dc28b6c 100644
--- a/content/common/input/event_with_latency_info.cc
+++ b/content/common/input/event_with_latency_info.cc
@@ -254,9 +254,10 @@ void Coalesce(const blink::WebInputEvent& event_to_coalesce,
 }  // namespace internal
 
 ScopedWebInputEventWithLatencyInfo::ScopedWebInputEventWithLatencyInfo(
-    const WebInputEvent& event,
+    ScopedWebInputEvent event,
     const ui::LatencyInfo& latency_info)
-    : event_(WebInputEventTraits::Clone(event)), latency_(latency_info) {}
+    : event_(std::move(event)), latency_(latency_info) {
+}
 
 ScopedWebInputEventWithLatencyInfo::~ScopedWebInputEventWithLatencyInfo() {}