Refactor wheel event handler registration in ScrollingCoordinator

This patch simplifies the process of observing wheel event handlers in
the ScrollingCoordinator. We do not need to compute exact counts of
handlers but instead check whether there are any or not.

We also add an API for indicating the presence of scroll event handlers.
This will be used by the compositor to schedule scrolling differently when
handlers are consuming scroll updates.

See https://codereview.chromium.org/206603002/ for full review history.

BUG=347366

Review URL: https://codereview.chromium.org/234913005

git-svn-id: svn://svn.chromium.org/blink/trunk@171506 bbb929c8-8fbe-4397-9dbb-9b2b20218538