[APC] Document/test actor hit-test coordinate spaces

APC geometry and actor tool targets use different coordinate spaces.
This CL documents the contract and adds coverage to prevent HiDPI
TOCTOU mismatches.

Viewport terminology:
- Layout viewport: viewport used for layout/scrolling.
- Visual viewport: what the user actually sees; can be offset from the
  layout viewport (e.g. when the mobile toolbar shows/hides during
  scrolling, or during pinch-zoom).

Pixel terminology:
- DIPs: device-independent pixels, aka CSS/logical pixels used
  by layout/JS APIs like getBoundingClientRect(). Example:
  DSF=2 means (1,1) DIP corresponds to (2,2) device pixels.
- Blink space: device pixels in the local root; sometimes
  referred to as "Blink pixels" in variable names.

Who uses what:
- APC node geometry (AIPageContentGeometry.*_bounding_box) is expressed
  in the local root's Blink space in visual-viewport coordinates.
- Tool coordinate targets (actor.mojom.ToolTarget.coordinate_dip) are
  expressed in DIPs in the local root's layout-viewport coordinates.
  DOM getBoundingClientRect() also uses CSS pixels (DIPs).

Callers comparing a DIP coordinate to APC geometry must convert from
layout-viewport DIPs to visual-viewport Blink space (e.g. apply the
appropriate device scale factor and any layout-vs-visual viewport
offset).

This clarifies expectations across actor tooling, optimization_guide
hit testing, and Blink APC geometry docs, and adds a regression test to
catch future coordinate-space mixups that can cause TOCTOU failures
(e.g. kObservedTargetElementChanged).

Bug: 464034559

Change-Id: I4c68bf4f02b13730f56a7667482948cfd9484aeb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7510297
Reviewed-by: Abigail Klein <abigailbklein@google.com>
Commit-Queue: Aaron Leventhal <aleventhal@google.com>
Reviewed-by: David Bokan <bokan@chromium.org>
Reviewed-by: Giovanni Ortuno Urquidi <ortuno@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1584293}