Set the unique_event_id when converting from TouchEvent to WebTouchEvent

ui/events/base_event_utils.h

+// Copyright 2015 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.
+
+#ifndef UI_EVENTS_BASE_EVENT_UTILS_H_
+#define UI_EVENTS_BASE_EVENT_UTILS_H_
+
+#include "base/basictypes.h"
+#include "ui/events/events_base_export.h"
+
+// Common functions to be used for all platforms.
+namespace ui {
+
+// Generate an unique identifier for events.
+EVENTS_BASE_EXPORT uint32 GetNextTouchEventId();
+
+}  // namespace ui
+
+#endif  // UI_EVENTS_BASE_EVENT_UTILS_H_