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This change refactors the logic to convert an ax node ID to a fuchsia semantic node ID.

It also fixes the bug where if a regular ax node had ID == 0, it would conflict with the root ID.

R=yangsharon

TEST: web_engine_unittests
Change-Id: Ie4ee52c65fbd5ad623986e015ad521e66c5e67fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2430264Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Sharon Yang <yangsharon@chromium.org>
Commit-Queue: Lucas Radaelli <lucasradaelli@google.com>
Cr-Commit-Position: refs/heads/master@{#811926}