Add missing return types for function parameters in ui/

These are required for the upcoming TypeScript 3.9.2 upgrade.

[email protected]

Bug: 1011811
Change-Id: Ibb7000f4c70863549bf6ce160a9fb89cb163e6f7
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2199122
Commit-Queue: Jack Franklin <[email protected]>
Reviewed-by: Jack Franklin <[email protected]>
Auto-Submit: Tim van der Lippe <[email protected]>
diff --git a/front_end/ui/ResizerWidget.js b/front_end/ui/ResizerWidget.js
index 418c642..dd1352a 100644
--- a/front_end/ui/ResizerWidget.js
+++ b/front_end/ui/ResizerWidget.js
@@ -105,8 +105,9 @@
     if (!this._elements.has(element)) {
       return false;
     }
-    elementDragStart(
-        element, this._dragStart.bind(this), this._drag.bind(this), this._dragEnd.bind(this), this.cursor(), event);
+    elementDragStart(element, this._dragStart.bind(this), event => {
+      this._drag(event);
+    }, this._dragEnd.bind(this), this.cursor(), event);
   }
 
   /**