Keep track in the browser of which frames have onunload and onbeforeunload handlers.

content/common/frame_messages.h

 // 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.
 
 // IPC messages for interacting with frames.
 // Multiply-included message file, hence no include guard.
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 1597 matching lines @@
                     int /* active_match_ordinal */,
                     bool /* final_update */)
 
 // Sends hittesting data needed to perform hittesting on the browser process.
 IPC_MESSAGE_ROUTED1(FrameHostMsg_HittestData, FrameHostMsg_HittestData_Params)
 
 // Asks the browser to display the file chooser.  The result is returned in a
 // FrameMsg_RunFileChooserResponse message.
 IPC_MESSAGE_ROUTED1(FrameHostMsg_RunFileChooser, content::FileChooserParams)
 
+// Messages to signal the presence or absence of BeforeUnload or Unload handlers

[nasko, 2017/03/29 23:31:26]

nit: You use capitalization inconsistently for the name of the two events,
lowercase is the web platform version, so might as well stick with that.

[jam, 2017/03/30 14:53:23]

Done.

+// for a frame. |present| is true if there is at least one of the handlers for
+// the frame.
+IPC_MESSAGE_ROUTED1(FrameHostMsg_BeforeUnloadHandlersPresent,
+                    bool /* present */)
+IPC_MESSAGE_ROUTED1(FrameHostMsg_UnloadHandlersPresent, bool /* present */)
+
 #if BUILDFLAG(USE_EXTERNAL_POPUP_MENU)
 
 // Message to show/hide a popup menu using native controls.
 IPC_MESSAGE_ROUTED1(FrameHostMsg_ShowPopup,
                     FrameHostMsg_ShowPopup_Params)
 IPC_MESSAGE_ROUTED0(FrameHostMsg_HidePopup)
 
 #endif
 
 #if defined(OS_ANDROID)
@@ skipping 23 matching lines @@
                     int /* nfr_request_id */,
                     float /* distance */)
 
 IPC_MESSAGE_ROUTED0(FrameHostMsg_NavigationHandledByEmbedder)
 #endif
 
 // Adding a new message? Stick to the sort order above: first platform
 // independent FrameMsg, then ifdefs for platform specific FrameMsg, then
 // platform independent FrameHostMsg, then ifdefs for platform specific
 // FrameHostMsg.