[omnibox][webui] Resize popup widget dynamically with web content

This CL changes some CSS styles and some native Views presenter logic
to get the omnibox popup resizing dynamically according to the size
of the autocomplete result set. It's somewhat better than the fixed
size with a lot of blank space, and it may be useful to be able to
resize as done here, but some problems still remain. Of note:
- Visual updates aren't synchronized between the browser UI/Views
  process and the webui renderer process. More detail added in a TODO.
- The bottom isn't rounded properly, and some extra bits of html/body
  can sometimes be rendered over frame decorations.

Some of these issues will require a fundamentally different approach
to the webui popup rendering, which will be explored in future CLs,
but this CL might be as good as we're likely to get with a hybrid
rendering approach involving webui content plus views frame decoration.

Bug: 1396174
Change-Id: Ie3785be010a9ebc8733e70ed4d0849bb5a656dbd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4742972


Code-Coverage: Findit <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Commit-Queue: Orin Jaworski <orinj@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1180429}