Cleanup: Pass std::string as const reference if possible

content/common/gpu/gpu_channel_manager.cc

Index: content/common/gpu/gpu_channel_manager.cc
diff --git a/content/common/gpu/gpu_channel_manager.cc b/content/common/gpu/gpu_channel_manager.cc
index 9a2be1dc80745ac9d9520864cb7c321ab75343c5..93e1556cd5ee8c4904e2b7062efb9e94196e1857 100644
--- a/content/common/gpu/gpu_channel_manager.cc
+++ b/content/common/gpu/gpu_channel_manager.cc
@@ -243,7 +243,7 @@ void GpuChannelManager::OnUpdateValueState(
     it->second->HandleUpdateValueState(target, state);
 }
 
-void GpuChannelManager::OnLoadedShader(std::string program_proto) {
+void GpuChannelManager::OnLoadedShader(const std::string& program_proto) {
   if (program_cache())
     program_cache()->LoadProgram(program_proto);
 }