Allow webview guests to be resized manually

This adds the `disableguestresize` property for webviews to prevent the
webview guest from reacting to size changes of the webview element. This
also partially documents the `webContents.setSize` function in order to
manually control the webview guest size.

These two features can be combined to improve resize performance for
e.g. webviews that span the entire window. This greatly reduces the lag
described in #6905.

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

@@ -172,7 +172,8 @@ class WebViewImpl {
     resizeEvent.newWidth = newSize.width
     resizeEvent.newHeight = newSize.height
     this.dispatchEvent(resizeEvent)
-    if (this.guestInstanceId) {
+    if (this.guestInstanceId &&
+        !this.attributes[webViewConstants.ATTRIBUTE_DISABLEGUESTRESIZE].getValue()) {
       guestViewInternal.setSize(this.guestInstanceId, {
         normal: newSize
       })