Add isLoadingMainFrame method to WebContents.

Also switch `webContents.executeJavaScript` to check it instead of `isLoading`.
There doesn’t seem to be a reasonable public way to get this information out of Chromium, so it’s synthesized here based on WebContentsObserver callbacks.
Fixes #5183.

lib/renderer/web-view/web-view.js

@@ -335,6 +335,7 @@ var registerWebViewElement = function () {
     'loadURL',
     'getTitle',
     'isLoading',
+    'isLoadingMainFrame',
     'isWaitingForResponse',
     'stop',
     'reload',