Add kill switch mechanism

This code implements the kill switch mechanism in the extension. This is
explained in //src/killSwitch/README.md and in the design doc:
https://docs.google.com/document/d/1O5YV6_WcxwrUyz-lwHOSTfZ3oyIFWj2EQee0VuKkhaA/edit.

Bug: twpowertools:64

Change-Id: Ia993c78035bba7038aafd53d156f20954217e86f
diff --git a/src/optionsCommon.js b/src/optionsCommon.js
index 42983a5..27db635 100644
--- a/src/optionsCommon.js
+++ b/src/optionsCommon.js
@@ -108,13 +108,26 @@
     var featuresLink = document.querySelector('.features-link');
     featuresLink.href = getDocURL('features.md');
 
-    var profileIndicatorLink = document.getElementById('profileIndicatorMoreInfo');
+    var profileIndicatorLink =
+        document.getElementById('profileIndicatorMoreInfo');
     profileIndicatorLink.href = getDocURL('op_indicator.md');
   }
 
   chrome.storage.sync.get(null, function(items) {
     items = cleanUpOptions(items, false);
 
+    // If some features have been force disabled, communicate this to the user.
+    if (items?._forceDisabledFeatures &&
+        items._forceDisabledFeatures.length > 0) {
+      if (window.CONTEXT == 'options') {
+        document.getElementById('kill-switch-warning')
+            .removeAttribute('hidden');
+      }
+
+      // TODO(avm99963): show a message above each option that has been force
+      // disabled
+    }
+
     for (var entry of Object.entries(optionsPrototype)) {
       var opt = entry[0];
       var optMeta = entry[1];