AIPageContentAgent: filter duplicates from z-order calculation early.

When hit-testing the viewport, AIPageContentAgent collects a vector of
NodeIDs that were hit. This may contain duplicates due to multiple
passes over the same node while hit testing. These need to be filtered
out. The most correct approach is probably to keep the first occurrence
of each node, because it's more likely it was added in a later paint
phase, which is more representative of what the page actually looks like
to the user (or actor).

Today, we keep the *last* occurrence of each node, which can cause bugs
where containers appear to have a higher z-order than their DOM
children. This CL adds a flag-gated clause which will ensure that only
the first occurrence of each element is used for computing z-order.

Existing z-order tests have been moved into a new test fixture, and the flag has been applied to those as well.

Change-Id: Ib6e7ddbf2c3e56a00bc9c6888d863dbf8b371be8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7011016
Reviewed-by: Xiaochen Zhou <xiaochenzh@chromium.org>
Reviewed-by: Khushal Sagar <khushalsagar@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Andrew Verge <averge@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1527230}