Bug 1304531 - Remove CPOW usage in browser_web_protocol_handlers.js. …

…r=Felipe

This one isn't intermittent but I noticed it in the logs and it was free to
fix.

MozReview-Commit-ID: D56wiHn3IfL

--HG--
extra : rebase_source : d8078fc995249d39d6c0e838d394f1465a9a3684

uriloader/exthandler/tests/mochitest/browser_web_protocol_handlers.js

@@ -38,7 +38,7 @@ add_task(function*() {
   handlerSvc.store(protoInfo);
 
   // Middle-click a testprotocol link and check the new tab is correct
-  let link = browser.contentDocument.getElementById("link");
+  let link = "#link";
   const expectedURL = "https://example.com/foobar?uri=testprotocol%3Atest";
 
   let promiseTabOpened =