Prevent the caption bubble from disappearing while hovering over it

This CL prevents the caption bubble from disappearing due to inactivity
while the mouse is hovering over it. It also resets the inactivity timer
when the mouse leaves the caption bubble frame.

Bug: 1342709
Change-Id: I4b80543f40e92f85a3b8761a1fbdf8bb9ad721d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3770884
Commit-Queue: Evan Liu <evliu@google.com>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Abigail Klein <abigailbklein@google.com>
Cr-Commit-Position: refs/heads/main@{#1026358}