Reland "[a11y] Align AxBlockFlowIterator with AbstractInlineTextBox for text-only navigation"

This is a reland of commit 0f9e3dd5d526280fc8400a7acbbe939ab5b36e75

Original change's description:
> [a11y] Align AxBlockFlowIterator with AbstractInlineTextBox for text-only navigation
>
> This change modifies the AxBlockFlowIterator
> algorithm to exclusively target fragment items
> associated with LayoutObjects that represent text.
>
> Currently, the AbstractInlineTextBox algorithm
> enforces this restriction for determining valid
> next/previous on-line candidates. This alignment
> ensures a fair comparison between the two
> algorithms.
>
> This definition is crucial because accessibility
> requires pointing to another AxInlineTextBox as
> the previous/next object of an existing
> AxInlineTextBox. Non-textual LayoutObjects cannot
> have an AxInlineTextBox associated with them,
> necessitating this rule.
>
> This fixes the following test, when the AxBlockFlowIterator flag is on:
> $ testing/xvfb.py  out/Release/content_shell \
> --run-web-tests \
> --enable-crash-reporter \
> --force-renderer-accessibility \
> --crash-dumps-dir=/tmp/test_crashes  \
>  --vmodule=ax_debug_utils=2 \
> external/wpt/css/css-flexbox/alignment/flex-align-baseline-line-clamp-001.tentative.html
>
> Change-Id: I99af2d3964ffb751facd15e0c3fd1db368cbd0c9
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6081904
> Reviewed-by: Kevin Ellis <kevers@chromium.org>
> Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
> Commit-Queue: Lucas Radaelli <lucasradaelli@google.com>
> Cr-Commit-Position: refs/heads/main@{#1395000}

Change-Id: Iebea309c35045532893f35e0ab38990395a1cf5c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6088066
Reviewed-by: Lucas Radaelli <lucasradaelli@google.com>
Auto-Submit: Andreea Costinas <acostinas@google.com>
Owners-Override: Andreea Costinas <acostinas@google.com>
Commit-Queue: Kevin Ellis <kevers@chromium.org>
Reviewed-by: Kevin Ellis <kevers@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1398834}