Support scroll to zoom in TimelineOverview.

https://bugs.webkit.org/show_bug.cgi?id=125909

Reviewed by Joseph Pecoraro.

* UserInterface/TimelineOverview.js:
(WebInspector.TimelineOverview):
(WebInspector.TimelineOverview.prototype._handleWheelEvent):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@162409 268f45cc-cd09-0410-ab3c-d52691b4dbfc