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

gpu/command_buffer/service/gpu_tracer.cc

Index: gpu/command_buffer/service/gpu_tracer.cc
diff --git a/gpu/command_buffer/service/gpu_tracer.cc b/gpu/command_buffer/service/gpu_tracer.cc
index 5ece0d4e72d90ca885093ce6e7d7bcab9a8cac46..049f10eccfe1989161ddbd56c7462ab55966f1d2 100644
--- a/gpu/command_buffer/service/gpu_tracer.cc
+++ b/gpu/command_buffer/service/gpu_tracer.cc
@@ -43,6 +43,8 @@ TraceMarker::TraceMarker(const std::string& category, const std::string& name)
       trace_(NULL) {
 }
 
+TraceMarker::TraceMarker(const TraceMarker& other) = default;
+
 TraceMarker::~TraceMarker() {
 }