Fix link-hints failing to launch.
Fix link-hints failing to launch.

Sometimes, link-hints mode fails to launch.  See Issue 1 from this post:
https://github.com/philc/vimium/issues/2081#issuecomment-210980903.

Here's a reproducible case:

- visit twitter
- using the vomnibar, visit any other page (in the same tab)
- hit `f` - the link hints fail to load.

What's happening is that the unregister/register frame messages for the
main/top frame arrive in the wrong order (first register, then
unregister).  Because these both have the same frame Id, the effect is
that the main/top frame ends up not registered.  So there are no
registered frames, so link hints mode doesn't launch.

Only the main/top frame has a re-usable frameId (`0`).  All other frames
receive a unique frame Id (which is never re-used).

Here, we just never unregister the main/top frame.  That way, it doesn't
matter which order the register/unregister messages arrive in.  If the
tab is navigating to a new page, then there'll be a new main/top frame
along soon.  If the tab is closing, then we tidy up in the
`chrome.tabs.onRemoved` handler.

philc/vimium,mrmr1993/vimium,smblott-github/vimium,mrmr1993/vimium,willsALMANJ/vimium,gdh1995/vimium,poacher2k/vimium,smblott-github/vimium,mrmr1993/vimium,smblott-github/vimium,gdh1995/vimium,philc/vimium,poacher2k/vimium,poacher2k/vimium,decaffeinate-examples/vimium,decaffeinate-examples/vimium,willsALMANJ/vimium,gdh1995/vimium,willsALMANJ/vimium,philc/vimium,decaffeinate-examples/vimium