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

gpu/command_buffer/common/sync_token.cc

Index: gpu/command_buffer/common/sync_token.cc
diff --git a/gpu/command_buffer/common/sync_token.cc b/gpu/command_buffer/common/sync_token.cc
index c938fe0cb3b555d92f0b90debb989362441be3ba..b9bba8fdf7900d4a189531f61bc1f19b1f0ba5b8 100644
--- a/gpu/command_buffer/common/sync_token.cc
+++ b/gpu/command_buffer/common/sync_token.cc
@@ -22,4 +22,6 @@ SyncToken::SyncToken(CommandBufferNamespace namespace_id,
       command_buffer_id_(command_buffer_id),
       release_count_(release_count) {}
 
+SyncToken::SyncToken(const SyncToken& other) = default;
+
 }  // namespace gpu