[views_ax] Introduce ViewAccessibility setters for accessible description

This CL introduces ViewAccessibility::SetDescription. One that expects
a std::string, another a std::u16string as a convenience.

We'll replace the callers of OverrideDescription in the next CL, and
remove OverrideDescription quickly after (we need to perform the same
change in Microsoft Edge simultaneously to avoid conflicts or
regressions).

Bug: 325137417
Change-Id: I4992635360716f14027540205752d115a4774547
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5303787


Auto-Submit: Benjamin Beaudry <benjamin.beaudry@microsoft.com>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Benjamin Beaudry <benjamin.beaudry@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1262109}