add width to webview iframe (#15204)

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

@@ -62,6 +62,7 @@ class WebViewImpl {
   createInternalElement () {
     const iframeElement = document.createElement('iframe')
     iframeElement.style.flex = '1 1 auto'
+    iframeElement.style.width = '100%'
     iframeElement.style.border = '0'
     v8Util.setHiddenValue(iframeElement, 'internal', this)
     return iframeElement