Support If-Modified-Since in cross-site thread watcher requests.

src/Monitoring/ThreadWatcher.coffee

@@ -183,19 +183,15 @@ ThreadWatcher =
       else
         ThreadWatcher.status.textContent = "#{Math.round(ThreadWatcher.fetched / ThreadWatcher.requests.length * 100)}%"
       cb.apply @, args
-    if siteID is Site.hostname
+    ajax = if siteID is Site.hostname then $.ajax else CrossOrigin.ajax
     if force
       delete $.lastModified.ThreadWatcher?[url]
     req = $.whenModified(
       url,
       'ThreadWatcher',
       onloadend,
-        {timeout: $.MINUTE}
+      {timeout: $.MINUTE, ajax}
     )
-    else
-      req = CrossOrigin.ajax url,
-        onloadend: onloadend
-        timeout: $.MINUTE
     ThreadWatcher.requests.push req
 
   clearRequests: ->

src/platform/$.coffee

@@ -82,7 +82,7 @@ $.ajax = do ->
 # This saves a lot of bandwidth and CPU time for both the users and the servers.
 $.lastModified = {}
 $.whenModified = (url, bucket, cb, options={}) ->
-  {timeout} = options
+  {timeout, ajax} = options
   params = []
   # XXX https://bugs.chromium.org/p/chromium/issues/detail?id=643659
   params.push "s=#{bucket}" if $.engine is 'blink'
@@ -92,7 +92,7 @@ $.whenModified = (url, bucket, cb, options={}) ->
   headers = {}
   if (t = $.lastModified[bucket]?[url0])?
     headers['If-Modified-Since'] = t
-  r = $.ajax url, {
+  r = (ajax or $.ajax) url, {
     onloadend: ->
       ($.lastModified[bucket] or= {})[url0] = @getResponseHeader('Last-Modified')
       cb.call @