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

content/browser/renderer_host/render_widget_host_view_aura.h

Index: content/browser/renderer_host/render_widget_host_view_aura.h
diff --git a/content/browser/renderer_host/render_widget_host_view_aura.h b/content/browser/renderer_host/render_widget_host_view_aura.h
index 95601ab524e71a2c625151e8eb0c1f93cbbfac1a..4cc8427532213178242cb9b6d2f936f0678cb2a0 100644
--- a/content/browser/renderer_host/render_widget_host_view_aura.h
+++ b/content/browser/renderer_host/render_widget_host_view_aura.h
@@ -44,6 +44,10 @@ class Canvas;
 class Display;
 }
 
+namespace gpu {
+struct Mailbox;
+}
+
 namespace ui {
 class CompositorLock;
 class InputMethod;
@@ -360,12 +364,12 @@ class RenderWidgetHostViewAura
   // The common entry point for full buffer updates from renderer
   // and GPU process.
   void BuffersSwapped(const gfx::Size& size,
-                      const std::string& mailbox_name,
+                      const gpu::Mailbox& mailbox_name,
                       const BufferPresentedCallback& ack_callback);
 
   bool SwapBuffersPrepare(const gfx::Rect& surface_rect,
                           const gfx::Rect& damage_rect,
-                          const std::string& mailbox_name,
+                          const gpu::Mailbox& mailbox_name,
                           const BufferPresentedCallback& ack_callback);
 
   void SwapBuffersCompleted(