Add an example using webRequest.getSecurityInfo() (#362)

* Add an example using webRequest.getSecurityInfo()

* Don't collect root certs if isUntrusted===true

root-cert-stats/background.js

@@ -0,0 +1,38 @@
+"use strict";
+
+var rootCertStats = {};
+
+/*
+On an onHeadersReceived event, if there was a successful TLS connection
+established, fetch the root cert and look at its subject.
+
+If we haven't seen this subject before, add it. If we have, increment its stats.
+*/
+async function logRootCert(details) {
+  try {
+    let securityInfo = await browser.webRequest.getSecurityInfo(
+      details.requestId,
+      {"certificateChain": true}
+    );
+    if ((securityInfo.state == "secure" || securityInfo.state == "weak") &&
+        !securityInfo.isUntrusted) {
+      let rootName = securityInfo.certificates[securityInfo.certificates.length - 1].subject;
+      if (rootCertStats[rootName] === undefined) {
+        rootCertStats[rootName] = 1;
+      } else {
+        rootCertStats[rootName] = rootCertStats[rootName] + 1;
+      }
+    }
+  }
+  catch(error) {
+    console.error(error);
+  }
+}
+
+/*
+Listen for all onHeadersReceived events.
+*/
+browser.webRequest.onHeadersReceived.addListener(logRootCert,
+  {urls: ["<all_urls>"]},
+  ["blocking"]
+);