Docserver: Update Future.Then() to be more Promise-like

chrome/common/extensions/docs/server2/fake_url_fetcher.py

Index: chrome/common/extensions/docs/server2/fake_url_fetcher.py
diff --git a/chrome/common/extensions/docs/server2/fake_url_fetcher.py b/chrome/common/extensions/docs/server2/fake_url_fetcher.py
index dbfaa5557ee4a352c6eec2bfb1a20a1495897b98..21b6cf6eb3f9a7b537ed06f95b5720d848cf7e96 100644
--- a/chrome/common/extensions/docs/server2/fake_url_fetcher.py
+++ b/chrome/common/extensions/docs/server2/fake_url_fetcher.py
@@ -129,11 +129,10 @@ class MockURLFetcher(object):
 
   def FetchAsync(self, url, **kwargs):
     self._fetch_async_count += 1
-    future = self._fetcher.FetchAsync(url, **kwargs)
-    def resolve():
+    def next(result):
       self._fetch_resolve_count += 1
-      return future.Get()
-    return Future(callback=resolve)
+      return result
+    return self._fetcher.FetchAsync(url, **kwargs).Then(next)
 
   def CheckAndReset(self,
                     fetch_count=0,