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Many Mac accessibility API use an AXTextMarker, which is an
opaque application-specific struct representing a position in
a document. Chrome implements these using AXPosition
(in particular a subclass BrowserAccessibilityPosition).

The code used the word "serialized" but it was actually
copying the raw bytes from a BrowserAccessibilityPosition
and then constructing a new C++ object with these bytes
later, which isn't defined behavior and may be causing
problems on macOS 10.15.

Instead, implement actual Serialize and Unserialize methods
on AXPosition and make use of those instead.

Tested with a unit test of AXPosition (based on the existing
test of Clone) and by manually manipulating some editable
text in Chrome with VoiceOver running, to ensure the code is
working correctly.

Bug: 998896
Change-Id: Ifb64b19ebdc96a8d36522456c77bca23cb58d552
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1776901
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#691970}