Makes the ontouch* events on window optional.

Browsers without touch support (e.g., Firefox/Chrome on desktop) will not have `ontouch*` events defined on `window`, so the current type definition is not correct.

Author: Micah Zoltu

baselines/dom.generated.d.ts

@@ -6061,10 +6061,10 @@ interface GlobalEventHandlers {
      */
     ontimeupdate: ((this: GlobalEventHandlers, ev: Event) => any) | null;
     ontoggle: ((this: GlobalEventHandlers, ev: Event) => any) | null;
-    ontouchcancel: ((this: GlobalEventHandlers, ev: TouchEvent) => any) | null;
-    ontouchend: ((this: GlobalEventHandlers, ev: TouchEvent) => any) | null;
-    ontouchmove: ((this: GlobalEventHandlers, ev: TouchEvent) => any) | null;
-    ontouchstart: ((this: GlobalEventHandlers, ev: TouchEvent) => any) | null;
+    ontouchcancel?: ((this: GlobalEventHandlers, ev: TouchEvent) => any) | null;
+    ontouchend?: ((this: GlobalEventHandlers, ev: TouchEvent) => any) | null;
+    ontouchmove?: ((this: GlobalEventHandlers, ev: TouchEvent) => any) | null;
+    ontouchstart?: ((this: GlobalEventHandlers, ev: TouchEvent) => any) | null;
     ontransitioncancel: ((this: GlobalEventHandlers, ev: TransitionEvent) => any) | null;
     ontransitionend: ((this: GlobalEventHandlers, ev: TransitionEvent) => any) | null;
     ontransitionrun: ((this: GlobalEventHandlers, ev: TransitionEvent) => any) | null;