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.

lib/browser/api/browser-window.js

@@ -3,6 +3,7 @@
 const {ipcMain} = require('electron')
 const {EventEmitter} = require('events')
 const {BrowserWindow} = process.atomBinding('window')
+const v8Util = process.atomBinding('v8_util')
 
 Object.setPrototypeOf(BrowserWindow.prototype, EventEmitter.prototype)
 
@@ -26,6 +27,34 @@ BrowserWindow.prototype._init = function () {
     ipcMain.emit('ELECTRON_GUEST_WINDOW_MANAGER_WINDOW_OPEN', event, url, frameName, disposition, options)
   })
 
+  this.webContents.on('-web-contents-created', (event, webContents, url,
+                                                frameName) => {
+    v8Util.setHiddenValue(webContents, 'url-framename', {url, frameName})
+  })
+  // Create a new browser window for the native implementation of
+  // "window.open"(sandbox mode only)
+  this.webContents.on('-add-new-contents', (event, webContents, disposition,
+                                            userGesture, left, top, width,
+                                            height) => {
+    let urlFrameName = v8Util.getHiddenValue(webContents, 'url-framename')
+    if ((disposition !== 'foreground-tab' && disposition !== 'new-window') ||
+        !urlFrameName) {
+      return
+    }
+
+    let {url, frameName} = urlFrameName
+    v8Util.deleteHiddenValue(webContents, 'url-framename')
+    const options = {
+      show: true,
+      x: left,
+      y: top,
+      width: width || 800,
+      height: height || 600,
+      webContents: webContents
+    }
+    ipcMain.emit('ELECTRON_GUEST_WINDOW_MANAGER_WINDOW_OPEN', event, url, frameName, disposition, options)
+  })
+
   // window.resizeTo(...)
   // window.moveTo(...)
   this.webContents.on('move', (event, size) => {