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* Now that every request is made by XProto,
  x11::Connection::Request::sequence is no longer needed.  Now sequences
  can be determined by (their index in the queue) +
  x11::Connection::first_request_id_.
  * This makes it easier to work with requests out-of-order, which will
    be necessary for creating property/window caches in the future.
* Move some xcb calls from xproto_types.cc into connection.cc.  I want
  to dlopen() libxcb in the future to make it possible to run Chrome on
  systems without any X11 libraries, so having everything consolidated
  will make that easier.
* Add Future::Wait().  Similar to Sync(), this blocks until a response
  is received, but instead of returning the response, it calls the
  installed response handler out-of-order.

R=sky

Bug: 739898
Change-Id: I10c4c18d3c3090ba09d660a5090fd7c0f01830f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2565440
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834386}