Bug 1780931 - [devtools] Set color-mix pref in browser_outputparser.js. r=jdescottes.

The test is permafailing on beta because it was missing
the color-mix pref, which is only enabled on Nightly.

Differential Revision: https://phabricator.services.mozilla.com/D152614

devtools/client/shared/test/browser_outputparser.js

@@ -10,6 +10,7 @@ const {
 add_task(async function() {
   await pushPref("layout.css.backdrop-filter.enabled", true);
   await pushPref("layout.css.individual-transform.enabled", true);
+  await pushPref("layout.css.color-mix.enabled", true);
   await addTab("about:blank");
   await performTest();
   gBrowser.removeCurrentTab();