Add more use counters for document.createTouch

Safari has 7 required arguments for document.createTouch and treats the
first two arguments as nullable. The hoped-for outcome is that Blink
can also have 7 required arguments, but that at least the target argument
is non-nullable. If not, Touch.prototype.target must be made nullable too.

BUG=647693
R=rbyers@chromium.org

Review-Url: https://codereview.chromium.org/2352333002
Cr-Commit-Position: refs/heads/master@{#422071}
5 files changed