dawn_wire: Reject new callbacks if the client is disconnected

If the wire client is disconnected, it will not receive any
messages from the server. Reject all callbacks that are created.

Bug: dawn:556
Change-Id: I2eb2c449b1ca6c8ea3e74040ef095abfc46a9061
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/31161
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: Stephen White <senorblanco@chromium.org>

src/dawn_wire/client/Client.cpp

@@ -83,6 +83,7 @@ namespace dawn_wire { namespace client {
     }
 
     void Client::Disconnect() {
+        mDisconnected = true;
         mSerializer = ChunkedCommandSerializer(NoopCommandSerializer::GetInstance());
         if (mDevice != nullptr) {
             mDevice->HandleDeviceLost("GPU connection lost");
@@ -94,4 +95,8 @@ namespace dawn_wire { namespace client {
         mDevices.Append(device);
     }
 
+    bool Client::IsDisconnected() const {
+        return mDisconnected;
+    }
+
 }}  // namespace dawn_wire::client