Include mojo/public/cpp/bindings/binding.h wherever mojo::Binding<> is used.

Currently, these places are picking it up via some include chain
involving interface_impl.h, which is bad since I'm going to delete that
file (since InterfaceImpl<> is deprecated and no longer used) -- I'll do
this separately.

R=rudominer@chromium.org

Review URL: https://codereview.chromium.org/1214633009 .

viewer/compositor/surface_holder.h

@@ -8,6 +8,7 @@
 #include "base/callback_forward.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/memory/weak_ptr.h"
+#include "mojo/public/cpp/bindings/binding.h"
 #include "mojo/services/surfaces/public/interfaces/surface_id.mojom.h"
 #include "mojo/services/surfaces/public/interfaces/surfaces.mojom.h"
 #include "ui/gfx/geometry/rect.h"

viewer/internals.h

@@ -8,6 +8,7 @@
 #include "base/memory/weak_ptr.h"
 #include "base/supports_user_data.h"
 #include "dart/runtime/include/dart_api.h"
+#include "mojo/public/cpp/bindings/binding.h"
 #include "mojo/public/interfaces/application/shell.mojom.h"
 #include "sky/services/testing/test_harness.mojom.h"