[User Education] Use acceleration for help bubble widgets on Linux

See attached bug. Help bubbles are cut off/not rendered when anchored to smaller windows on Linux (and probably Lacros) because they are not accelerated by default. Enabling acceleration allows them to float properly and attach to e.g. menus without graphical glitches.

Also improves some tests around help bubbles to make sure we don't cause any regressions with this change. The specific case is not tested for regression due to it being nearly impossible to access the relevant information in the widget.

Follow-up:
 - make sure that focusing a help bubble attached to a menu does not
   inadvertently close the menu (we already have this facility for
   bubble dialogs)

Bug: 1445770
Change-Id: I842a1d3ea8ea8b29a6b503a2bd3957dc8bdc5ee6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4534412
Reviewed-by: Keren Zhu <kerenzhu@chromium.org>
Code-Coverage: Findit <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Dana Fried <dfried@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1145616}