Unshare logic between mousemove/mouseleave event processing again.

I tried to unify this in r154247 to get rid of a redundant protective
RefPtr. However, this caused some issues with interactions between
context menus and tooltips on OS X. This is mostly a revert but with
some of the members renamed to make the naming more consistent/clearer.

BUG=261132

Review URL: https://chromiumcodereview.appspot.com/22897016

git-svn-id: svn://svn.chromium.org/blink/trunk@156797 bbb929c8-8fbe-4397-9dbb-9b2b20218538