gpu: Add out-of-line copy ctors for complex classes.

gpu/command_buffer/common/sync_token.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "gpu/command_buffer/common/sync_token.h"
 
 namespace gpu {
 
 SyncToken::SyncToken()
     : verified_flush_(false),
       namespace_id_(CommandBufferNamespace::INVALID),
       extra_data_field_(0),
       release_count_(0) {}
 
 SyncToken::SyncToken(CommandBufferNamespace namespace_id,
                      int32_t extra_data_field,
                      CommandBufferId command_buffer_id,
                      uint64_t release_count)
     : verified_flush_(false),
       namespace_id_(namespace_id),
       extra_data_field_(extra_data_field),
       command_buffer_id_(command_buffer_id),
       release_count_(release_count) {}
 
+SyncToken::SyncToken(const SyncToken& other) = default;
+
 }  // namespace gpu