[a11y] Fix accessibility of multi-line list markers

This CL fixes an issue where the accessibility of list markers was broken for
multi-line list items.

This CL adds a check to ensure that for the first inline text box of a list
item, the `PreviousOnLine` method returns the list marker, ut not for the other
AXInlineTextBoxes, as they are either in different lines or part of the same
line as the first AXInlineTextBox.

A new browser test is added to verify the fix.

Bug: 441113360
Change-Id: Idadd412a9811d26dcc79b56e03a999108d233684
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6917313
Reviewed-by: Kevin Ellis <kevers@chromium.org>
Commit-Queue: Lucas Radaelli <lucasradaelli@google.com>
Cr-Commit-Position: refs/heads/main@{#1511555}