Flush WebGPU commands after fence.onCompletion

Currently, this callback is processed entirely on the client, so it
doesn't require a flush. It will be updated to be server-side for
consistent callback ordering. In order to ensure the server receives
the callback in finite time, we force a flush after the command is
sent.

Bug: dawn:516
Change-Id: Ibfa64b142331663e818c1142021ec2db1eeb9f58
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2380026Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Austin Eng <enga@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802317}

third_party/blink/renderer/modules/webgpu/gpu_fence.cc

@@ -4,6 +4,7 @@
 
 #include "third_party/blink/renderer/modules/webgpu/gpu_fence.h"
 
+#include "gpu/command_buffer/client/webgpu_interface.h"
 #include "third_party/blink/renderer/bindings/core/v8/script_promise_resolver.h"
 #include "third_party/blink/renderer/core/dom/dom_exception.h"
 #include "third_party/blink/renderer/modules/webgpu/dawn_callback.h"
@@ -56,6 +57,10 @@ ScriptPromise GPUFence::onCompletion(ScriptState* script_state,
   GetProcs().fenceOnCompletion(GetHandle(), value, callback->UnboundCallback(),
                                callback->AsUserdata());
 
+  // WebGPU guarantees that submitted commands finish in finite time so we
+  // flush commands to the GPU process now.
+  device_->GetInterface()->FlushCommands();
+
   return promise;
 }