[X11] Avoid generating mouse exited events for pointer grabs

Some WMs (in particular Fluxbox) capture the pointer during clicks.
This CL avoids generating a mouse exited event for this case.

R=sky

Change-Id: If36f42e55e548fb41c3c0f6a19d4466c01a1df76
Fixed: 41314367
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5787625
Reviewed-by: Scott Violet <sky@chromium.org>
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1342261}