Enable no-global-assign lint rule

This actually caught an issue in ElementsTestRunner.js.

Bug: chromium:1057042
Change-Id: I850156e6b7956fe9b24295eb084e5b228113bda9
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2201178
Reviewed-by: Tim van der Lippe <[email protected]>
Reviewed-by: Yang Guo <[email protected]>
Commit-Queue: Mathias Bynens <[email protected]>
diff --git a/front_end/animation/animation-legacy.js b/front_end/animation/animation-legacy.js
index 4c65c7b..7c46b45 100644
--- a/front_end/animation/animation-legacy.js
+++ b/front_end/animation/animation-legacy.js
@@ -5,6 +5,7 @@
 import * as AnimationModule from './animation.js';
 
 self.Animation = self.Animation || {};
+/* global Animation:writable */
 Animation = Animation || {};
 
 /**