Add XHR proxy kill switch

Since the XHRProxy loads very early to be able to catch all requests,
we cache the kill switch value in the localStorage and this cached
value is consulted early in the page load to determine whether the
XHR proxy should be put in place.

Thus, this kill switch only takes effect after a page reload.

Fixed: twpowertools:162
Change-Id: I43a163c506657d5ba9fb4ec268b2d4409d6401e3
diff --git a/src/common/optionsPrototype.json5 b/src/common/optionsPrototype.json5
index 1b114a4..eb7d892 100644
--- a/src/common/optionsPrototype.json5
+++ b/src/common/optionsPrototype.json5
@@ -174,6 +174,12 @@
     killSwitchType: 'ignore',
   },
 
+  // Internal kill switches:
+  'killswitch_xhrproxy': {
+    context: 'internal',
+    killSwitchType: 'internalKillSwitch',
+  },
+
   // Deprecated options:
   'escalatethreads': {
     defaultValue: false,