From comments on https://codereview.chromium.org/2345663002/

Of course, you don't have push_handle_ yet, but maybe something like this
refactor:

  o rename TryHandle to PushHandle
      o in addition to Cancel(), it also has Try()
Refactor the current QuicClientPushPromiseIndex::Try() so into two bits:
  o new method QuicClientPushPromiseIndex::Lookup() (returns PushHandle)
           o using this, found_promised_ could be consolidated into push_handle_
  o remainder of Try() logic goes into new PushHandle::Try() method