Move client-side OnCompletion callbacks to the server.

We need callbacks to be processed server-side so that callback
ordering can be made consistent.

Bug: dawn:516
Change-Id: Ie5590ca33fce6bda431f93ae9ff8e832468109c1
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/27481
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>

src/dawn_wire/server/ServerQueue.cpp

@@ -21,7 +21,6 @@ namespace dawn_wire { namespace server {
         if (cFence == nullptr) {
             return false;
         }
-
         mProcs.queueSignal(cSelf, cFence, signalValue);
 
         ObjectId fenceId = FenceObjectIdTable().Get(cFence);