Bug 1692140 - Fix talos test pdfpaint measurement. r=emilio,perftest-reviewers

Now that we delay the load event for pdf.js, the pagerendered event listener
was being added too late and we were catching the second page rendered event.
Also, add a check to ensure the page rendered is the first page.

Differential Revision: https://phabricator.services.mozilla.com/D104866

testing/talos/talos/pageloader/chrome/lh_pdfpaint.js

@@ -5,6 +5,12 @@ function _pdfPaintHandler() {
   content.window.addEventListener(
     "pagerendered",
     e => {
+      if (e.detail.pageNumber !== 1) {
+        sendAsyncMessage("PageLoader:Error", {
+          msg: `Error: Expected page 1 got ${e.detail.pageNumber}`,
+        });
+        return;
+      }
       sendAsyncMessage("PageLoader:LoadEvent", {
         time: e.detail.timestamp,
         name: "pdfpaint",
@@ -15,7 +21,7 @@ function _pdfPaintHandler() {
 }
 
 addEventListener(
-  "load",
+  "DOMContentLoaded",
   // eslint-disable-next-line no-undef
   contentLoadHandlerCallback(_pdfPaintHandler),
   true