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

content/common/frame_messages.h

Index: content/common/frame_messages.h
diff --git a/content/common/frame_messages.h b/content/common/frame_messages.h
index f2b13b701c9f9351cdbd845d418c3f11b4716579..59beffbfb5ff8dabb08365c4df40b408c84fa245 100644
--- a/content/common/frame_messages.h
+++ b/content/common/frame_messages.h
@@ -1615,6 +1615,13 @@ IPC_MESSAGE_ROUTED1(FrameHostMsg_HittestData, FrameHostMsg_HittestData_Params)
 // 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.