[Autofill Assistant] Have click check whether element is on top.

With this change, the click action can optionally check whether an
element is on top and either report its finding, but click anyways or
not click.

This is implemented as three web actions:

- WaitUntilElementIsStable
- CheckOnTop (optional)
- ClickOrTap

Three actions are needed as CheckOnTop must be executed after the
element has stopped moving but before actually clicking or tapping.

The logic for checking whether the element is on top is the same as the
one used by the on_top filter; the two share the same JavaScript
snippet.

Bug: b/171463353
Change-Id: I8ed71f3b6c07a9254d16d314220b008669f4d600
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2517578


Commit-Queue: Stephane Zermatten <szermatt@chromium.org>
Reviewed-by: Clemens Arbesser <arbesser@google.com>
Reviewed-by: Sandro Maggi <sandromaggi@google.com>
Reviewed-by: Marian Fechete <marianfe@google.com>
Cr-Commit-Position: refs/heads/master@{#824883}