Bug 1528762. Don't inject customElements.js into system-principal about:blank. r=bgrins

Otherwise we get a performance regression on tp5o responsiveness and
tp5o_webext responsiveness when about:blank starts firing
document-element-inserted notifications.

Differential Revision: https://phabricator.services.mozilla.com/D20873

--HG--
extra : moz-landing-system : lando

toolkit/components/processsingleton/CustomElementsListener.jsm

@@ -15,7 +15,11 @@ Services.obs.addObserver({
       doc.contentType == "application/vnd.mozilla.xul+xml" ||
       doc.contentType == "application/xhtml+xml" ||
       doc.contentType == "text/html"
-    )) {
+    ) &&
+        // People shouldn't be using our built-in custom elements in
+        // system-principal about:blank anyway, and trying to support that
+        // causes responsiveness regressions.  So let's not support it.
+        doc.URL != "about:blank") {
       Services.scriptloader.loadSubScript(
         "chrome://global/content/customElements.js", doc.ownerGlobal);
     }