Enable TypeScript for TransformController.js

Bug: 1011811
Change-Id: I79e2dadc932d78f2c63cabdb36db7f14feea564b
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2463360
Reviewed-by: Paul Lewis <[email protected]>
Commit-Queue: Alex Rudenko <[email protected]>
diff --git a/front_end/timeline/TimelinePaintProfilerView.js b/front_end/timeline/TimelinePaintProfilerView.js
index 01c91d9..fcb7311 100644
--- a/front_end/timeline/TimelinePaintProfilerView.js
+++ b/front_end/timeline/TimelinePaintProfilerView.js
@@ -167,7 +167,8 @@
     this._maskElement = this._imageContainer.createChild('div');
     this._imageElement.addEventListener('load', this._updateImagePosition.bind(this), false);
 
-    this._transformController = new LayerViewer.TransformController.TransformController(this.contentElement, true);
+    this._transformController = new LayerViewer.TransformController.TransformController(
+        /** @type {!HTMLElement} */ (this.contentElement), true);
     this._transformController.addEventListener(
         LayerViewer.TransformController.Events.TransformChanged, this._updateImagePosition, this);
   }