UseCounter for explicit mouse capture affecting click event target.

In PEWG, we are considering making the target of a click event the same
as the pointerup target when the pointerup event is dispatched to a
captured target.  Currently the click event target is the common
ancestor of pointerdown and pointerup targets.  Before we make the
change, we want to check what fraction of page loads would be affected
by this possible change in click target.

Spec discussion: https://github.com/w3c/pointerevents/issues/356

Fixed: 1199099
Change-Id: I9e28c9ab5138f187635352a74befb8cb800aa80a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2876145
Commit-Queue: Mustaq Ahmed <mustaq@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#893096}