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 bff9e8b6b85ba9bce20b8fc19687cf1df89ea491..ba0b8ee317f06e6a0ea4f908fec72bc1e3f8aec3 100644
--- a/ui/compositor/compositor.cc
+++ b/ui/compositor/compositor.cc
@@ -11,7 +11,6 @@
 #include "base/command_line.h"
 #include "base/memory/singleton.h"
 #include "base/message_loop.h"
-#include "base/string_util.h"
 #include "base/threading/thread.h"
 #include "base/threading/thread_restrictions.h"
 #include "cc/context_provider.h"
@@ -20,6 +19,7 @@
 #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"
@@ -285,8 +285,8 @@ 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)