Change Event.deepPath to a method

In order to make clear that computation is performed, Event.deepPath has been changed to a method Event.deepPath(), that returns sequence<EventTarget>.
See the discussion here: WICG/webcomponents#361
Described in the spec here : http://w3c.github.io/webcomponents/spec/shadow/#widl-Event-deepPath-sequence-EventTarget
See the spec change here: WICG/webcomponents@ed93413
BUG=531990

Review URL: https://codereview.chromium.org/1637813002

Cr-Commit-Position: refs/heads/master@{#372596}

front_end/ui/Tooltip.js

@@ -33,7 +33,7 @@ WebInspector.Tooltip.prototype = {
      */
     _mouseMove: function(event)
     {
-        var path = event.deepPath ? event.deepPath : event.path;
+        var path = event.deepPath() ? event.deepPath() : event.path;
         if (!path || event.buttons !== 0)
             return;