Bug 1577988 - Don't add toggle.svg to whitelist of unused files if running mochitest devtools. a=test-only-beta70-fix

Differential Revision: https://phabricator.services.mozilla.com/D44315

--HG--
extra : rebase_source : 41c6419e80f18410747a302cf78b6b0f9e98602b
extra : amend_source : 2891a5e0bca16e4d05a10e4657141370f56206f4

browser/base/content/test/static/browser_all_files_referenced.js

@@ -279,7 +279,7 @@ if (!isDevtools) {
   }
 }
 
-if (!AppConstants.NIGHTLY_BUILD) {
+if (!AppConstants.NIGHTLY_BUILD && !isDevtools) {
   // Bug 1532703 - only used in HTML-based about:config
   whitelist.add("chrome://browser/skin/toggle.svg");
 }