Bug 1568278 followup: fix incorrect 'let' usage on CLOSED TREE. r=bzbarsky

browser/base/content/browser.js

@@ -8156,7 +8156,7 @@ var CanvasPermissionPromptHelper = {
 
     let browser;
     if (aSubject instanceof Ci.nsIDOMWindow) {
-      let browser = aSubject.docShell.chromeEventHandler;
+      browser = aSubject.docShell.chromeEventHandler;
     } else {
       browser = aSubject;
     }