GitLab

This CL implements GetOffsetAtPoint which returns the offset of the
character located at the position and adds the unittest for the API.

AX-Relnotes: It supports atk_text_get_offset_at_point() on Linux.
Bug: 1070742
Change-Id: Iff075f3e4f95cc746b67819b9426300c250ff485
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2280805
Commit-Queue: Julie Kim <jkim@igalia.com>
Reviewed-by: Martin Robinson <mrobinson@igalia.com>
Cr-Commit-Position: refs/heads/master@{#785740}