Implement <webview> droplink event

content/browser/browser_plugin/browser_plugin_embedder.h

 // Copyright (c) 2012 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.
 
 // A BrowserPluginEmbedder handles messages coming from a BrowserPlugin's
 // embedder that are not directed at any particular existing guest process.
 // In the beginning, when a BrowserPlugin instance in the embedder renderer
 // process requests an initial navigation, the WebContents for that renderer
 // renderer creates a BrowserPluginEmbedder for itself. The
 // BrowserPluginEmbedder, in turn, forwards the requests to a
@@ skipping 24 matching lines @@
   static BrowserPluginEmbedder* Create(WebContentsImpl* web_contents);
 
   // Called when embedder's |rwh| has sent screen rects to renderer.
   void DidSendScreenRects();
 
   // WebContentsObserver implementation.
   bool OnMessageReceived(const IPC::Message& message) override;
   bool OnMessageReceived(const IPC::Message& message,
                          RenderFrameHost* render_frame_host) override;
 
+  // Sends a 'dragend' message to the guest that started the drag.
   void DragSourceEndedAt(int client_x, int client_y, int screen_x,
       int screen_y, blink::WebDragOperation operation);
 
-  void DragEnteredGuest(BrowserPluginGuest* guest);
+  // Indicates that a drag operation has entered into the bounds of a given
+  // |guest|. Returns whether the |guest| also started the operation.
+  bool DragEnteredGuest(BrowserPluginGuest* guest);
 
+  // Indicates that a drag operation has left the bounds of a given |guest|.
   void DragLeftGuest(BrowserPluginGuest* guest);
 
+  // Called by WebContentsViewGuest when a drag operation is started within
+  // |guest|. This |guest| will be signaled at the end of the drag operation.
   void StartDrag(BrowserPluginGuest* guest);
 
   // Sends EndSystemDrag message to the guest that initiated the last drag/drop
   // operation, if there's any.
   void SystemDragEnded();
 
   // Used to handle special keyboard events.
   bool HandleKeyboardEvent(const NativeWebKeyboardEvent& event);
 
   // Find the given |search_text| in the page. Returns true if the find request
@@ skipping 43 matching lines @@
   bool guest_drag_ending_;
 
   base::WeakPtrFactory<BrowserPluginEmbedder> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(BrowserPluginEmbedder);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_BROWSER_PLUGIN_BROWSER_PLUGIN_EMBEDDER_H_