Implement <webview> droplink event

content/browser/browser_plugin/browser_plugin_embedder.h

Index: content/browser/browser_plugin/browser_plugin_embedder.h
diff --git a/content/browser/browser_plugin/browser_plugin_embedder.h b/content/browser/browser_plugin/browser_plugin_embedder.h
index 7153fd15c880042f3d64e6a5d96ecb581c2dadfd..c42f7b84ed76ecde24e9e3c7e7feb682936b590f 100644
--- a/content/browser/browser_plugin/browser_plugin_embedder.h
+++ b/content/browser/browser_plugin/browser_plugin_embedder.h
@@ -42,13 +42,19 @@ class CONTENT_EXPORT BrowserPluginEmbedder : public WebContentsObserver {
   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