Can't undo in non-editor webviews

- VSCode Version: master
- OS Version: macOS

Steps to Reproduce:
1. open a normal (non-inline-editor) webview which contains an `<input>` element
2. Type into the input
3. Try to `cmd-z` to undo, but it doesn't do anything
4. same with `cmd-shift-z` to redo.

Does this issue occur when all extensions are disabled?: Yes, but you need an extension that provides a webview to repro.
