Use event-driven or direct script initialization

Chrome may inject the script immediately after the readystatechange/load events fired, so we need to explicitly check the readyState after script injection.
Also unconditionally listen to the window.onload event for further cross-browser robustness (we have init-once logic either way).

inject.js

@@ -193,17 +193,22 @@ chrome.extension.sendMessage({}, function(response) {
     if (blacklisted)
       return;
 
-    var readyStateCheckInterval = setInterval(function() {
-      if (document && document.readyState === 'complete') {
-        clearInterval(readyStateCheckInterval);
+    window.onload = () => initializeNow(document);
+    if (document) {
+      if (document.readyState === "complete") {
+        initializeNow(document);
+      } else {
+        document.onreadystatechange = () => {
+          if (document.readyState === "complete") {
             initializeNow(document);
           }
-    }, 10);
+        }
+      }
+    }
   }
 
   function initializeNow(document) {
-      // in theory, this should only run once, in practice..
-      // that's not guaranteed, hence we enforce own init-once.
+      // enforce init-once due to redundant callers
       if (document.body.classList.contains('vsc-initialized')) {
         return;
       }