Use gpu::Mailbox in IPCs instead of std::string

ui/compositor/compositor.cc

Index: ui/compositor/compositor.cc
diff --git a/ui/compositor/compositor.cc b/ui/compositor/compositor.cc
index 4cb912297bb40c16604125d0115971c9ae9e55ac..c628976f691f0a2166879b56e3f4cb58985d306d 100644
--- a/ui/compositor/compositor.cc
+++ b/ui/compositor/compositor.cc
@@ -20,6 +20,7 @@
 #include "cc/layer_tree_host.h"
 #include "cc/output_surface.h"
 #include "cc/thread_impl.h"
+#include "gpu/command_buffer/common/mailbox.h"
 #include "third_party/skia/include/core/SkBitmap.h"
 #include "ui/compositor/compositor_observer.h"
 #include "ui/compositor/compositor_switches.h"
@@ -262,8 +263,8 @@ Texture::Texture(bool flipped, const gfx::Size& size, float device_scale_factor)
 Texture::~Texture() {
 }
 
-std::string Texture::Produce() {
-  return EmptyString();
+gpu::Mailbox Texture::Produce() {
+  return gpu::Mailbox();
 }
 
 CompositorLock::CompositorLock(Compositor* compositor)