Fix issues with scrolling and frames

Fix issues with scrolling and frames

Since commit 4958beb3b we use `MutationObserver`s to keep
`vim.state.scrollableElements` up-to-date when scrollable elements are removed.
However, there are two more ways elements can be removed if the are located
inside a frame:

- If the whole frame is removed.
- If the `src` attribute of the frame is changed.

This caused "TypeError: can't access dead object" errors on gmail.com.

This commit takes care of those two frame cases as well.

See #605.