Can't undo in non-editor webviews

[evangrayk]

• 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.