Add support for native chromium popups on sandboxed renderers.

- Allow `api::Window` instances to be created from existing `api::WebContents`.
- Override `WebContentsCreated` and `AddNewContents` to wrap renderer-created
  `content::WebContents` into `api::WebContents`.
- For `content::WebContents` that should be displayed in new windows, pass the
  wrapped `api::WebContents` object to window manager.

atom/browser/api/atom_api_web_contents.h

@@ -210,6 +210,17 @@ class WebContents : public mate::TrackableObject<WebContents>,
                            const base::string16& message,
                            int32_t line_no,
                            const base::string16& source_id) override;
+  void WebContentsCreated(content::WebContents* source_contents,
+                          int opener_render_frame_id,
+                          const std::string& frame_name,
+                          const GURL& target_url,
+                          content::WebContents* new_contents) override;
+  void AddNewContents(content::WebContents* source,
+                      content::WebContents* new_contents,
+                      WindowOpenDisposition disposition,
+                      const gfx::Rect& initial_rect,
+                      bool user_gesture,
+                      bool* was_blocked) override;
   content::WebContents* OpenURLFromTab(
       content::WebContents* source,
       const content::OpenURLParams& params) override;