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

gpu/command_buffer/service/mailbox_manager_sync.cc

Index: gpu/command_buffer/service/mailbox_manager_sync.cc
diff --git a/gpu/command_buffer/service/mailbox_manager_sync.cc b/gpu/command_buffer/service/mailbox_manager_sync.cc
index d6cac104c7ea90b1690a32caaa458bdd67cc849f..8cf799232fa9dd52e0853f102108dbefaf53ec0b 100644
--- a/gpu/command_buffer/service/mailbox_manager_sync.cc
+++ b/gpu/command_buffer/service/mailbox_manager_sync.cc
@@ -166,6 +166,9 @@ MailboxManagerSync::TextureGroupRef::TextureGroupRef(unsigned version,
     : version(version), group(group) {
 }
 
+MailboxManagerSync::TextureGroupRef::TextureGroupRef(
+    const TextureGroupRef& other) = default;
+
 MailboxManagerSync::TextureGroupRef::~TextureGroupRef() {
 }