Update root-cert-stats to use runtime.sendMessage #542 (#547)

* Adding send message and on message functions

* moving table displaying into a new function

* updating rootCertStats from var to let

* updating comments

* Apply Linter change, unused const

Co-authored-by: Rob Wu <rob@robwu.nl>

---------

Co-authored-by: Rob Wu <rob@robwu.nl>
Co-authored-by: rebloor <git@sherpa.co.nz>

root-cert-stats/background.js

@@ -1,9 +1,6 @@
 "use strict";
 
-// Note: declared with "var" because popup.js references this global variable.
-// If this were to be declared with "const" or "let", then the variable would
-// still be available to this file, but not to popup.js.
-var rootCertStats = {};
+let rootCertStats = {};
 
 /*
 On an onHeadersReceived event, if there was a successful TLS connection
@@ -39,3 +36,12 @@ browser.webRequest.onHeadersReceived.addListener(logRootCert,
   {urls: ["<all_urls>"]},
   ["blocking"]
 );
+
+/*
+Send the rootCertStats object to popup.js when requested.
+*/
+browser.runtime.onMessage.addListener((message, sender, sendResponse) => {
+  if (message.action === "getRootCertStats") {
+    sendResponse({ rootCertStats: rootCertStats });
+  }
+});