Bug 1469191 - Use the proper window for synthesizeMouseAtCenter in netmonitor test;r=jdescottes

It was using `window` which refers to the browser window, but we actually want to synthesize
the click in the devtools window.

MozReview-Commit-ID: KeIwq3JIJin

--HG--
extra : rebase_source : c769229d19661370d5448f0b5b236ce8b2eb91c4

devtools/client/netmonitor/test/browser_net_filter-autocomplete.js

@@ -51,7 +51,7 @@ add_task(async function() {
   await waitNetwork;
 
   EventUtils.synthesizeMouseAtCenter(
-    document.querySelector(".devtools-filterinput"), {}, window);
+    document.querySelector(".devtools-filterinput"), {}, document.defaultView);
   // Empty Mouse click should keep autocomplete hidden
   ok(!document.querySelector(".devtools-autocomplete-popup"),
     "Autocomplete Popup still hidden");