[XProto] Fix sequence_ not getting reset after calling OnResponse()

Fixes a regression introduced after [1] where Future would call
OnResponseImpl() in destruction even if it was already called.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/2220524

BUG=1066670
TBR=msisov

Change-Id: I6c42809ec3f5973c141eb76e52650aa560fda6d7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2227056Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774297}

ui/gfx/x/xproto_types.cc

@@ -52,16 +52,16 @@ FutureBase& FutureBase::operator=(FutureBase&& future) {
 }
 
 void FutureBase::SyncImpl(Error** raw_error, uint8_t** raw_reply) {
-  if (!sequence_)
-    return;
-
+  DCHECK(sequence_);
   *raw_reply = reinterpret_cast<uint8_t*>(
       xcb_wait_for_reply(connection_->XcbConnection(), *sequence_, raw_error));
   sequence_ = base::nullopt;
 }
 
 void FutureBase::OnResponseImpl(ResponseCallback callback) {
+  DCHECK(sequence_);
   connection_->AddRequest(*sequence_, std::move(callback));
+  sequence_ = base::nullopt;
 }
 
 }  // namespace x11