[Autofill Assistant] Make ElementFinder matches ordered.

This CL refactors ElementFinder so that the elements it matches have the
same order than in the document. This is done with 2 changes:

1) Elements matched by the css_selector filter are now added in the same
order than the parent element that was used to query the selector.
2) The results of the background tasks are now added in current_matches_
in the same order than the parent element that was used as the root of
the background task.

I will add relevant browser tests once we can easily check multiple
matching elements (i.e. either via a new pick_at filter or getting the
outerHTML of multiple elements).

Change-Id: If1f7098d3467461a39a3b34773f1e02db0a73069
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2517577


Commit-Queue: Jordan Demeulenaere <jdemeulenaere@chromium.org>
Reviewed-by: Sandro Maggi <sandromaggi@google.com>
Reviewed-by: Stephane Zermatten <szermatt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824789}