Cleanup: Pass std::string as const reference from gpu/

gpu/tools/compositor_model_bench/render_tree.h

Index: gpu/tools/compositor_model_bench/render_tree.h
diff --git a/gpu/tools/compositor_model_bench/render_tree.h b/gpu/tools/compositor_model_bench/render_tree.h
index 6b43fbe77f90f9d270ad1321c95818499ff572c3..5a460bc701cfb7d3e4aca9706dbf42553d4e9bd6 100644
--- a/gpu/tools/compositor_model_bench/render_tree.h
+++ b/gpu/tools/compositor_model_bench/render_tree.h
@@ -35,7 +35,7 @@ struct Texture {
   GLenum format;
 };
 
-GLenum TextureFormatFromString(std::string format);
+GLenum TextureFormatFromString(const std::string& format);
 const char* TextureFormatName(GLenum format);
 int FormatBytesPerPixel(GLenum format);