[inspector] Move a11y protocol type builders completely into helper file

This CL is a pure mechanical change that moves the "Build*" functions
into inspector_type_builder_helper.cc.

Additional drive-by changes:
  * Consistently use ax::mojom::blink::* enum types.
  * Move "AddChildren", "AddAncestors" and "WalkAXNodesToDepth" into
    anonymous namespace (instead of being class methods).

This CL prepares the InspectorAccessibilityAgent to be spilt into an
agent and a "per document delegate" (see https://crrev.com/c/6298248
for a large picture prototype ). The agent will move into core/ and
the delegate remains in "modules/accessibility". This will simplify
the life-time management of the InspectorAccessibilityAgent.

Bug: 398967289
Change-Id: I0b818b0bfc48082d6e54cb772464aa8cfc2d9005
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6299169
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Commit-Queue: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1424540}