buffet: Add ObjectManager interface to /org/chromium/Buffet

BUG=chromium:359190
TEST=`buffet_client GetManagedObjects` returns
message_type: MESSAGE_METHOD_RETURN
destination: :1.35
sender: :1.31
signature: a{oa{sa{sv}}}
serial: 10
reply_serial: 3

array [
  dict entry {
    object_path "/org/chromium/Buffet/Manager"
    array [
      dict entry {
        string "org.chromium.Buffet.Manager"
        array [
          dict entry {
            string "State"
            variant               string "{}"
          }
        ]
      }
    ]
  }
]

Done.

Change-Id: I2bbcc9a3f71c7ec6ab76cb4600dad7efe1a8bb0a
Reviewed-on: https://chromium-review.googlesource.com/198963
Reviewed-by: Chris Sosa <sosa@chromium.org>
Commit-Queue: Christopher Wiley <wiley@chromium.org>
Tested-by: Christopher Wiley <wiley@chromium.org>
Reviewed-by: Ilja Friedel <ihf@chromium.org>
diff --git a/buffet/exported_property_set.cc b/buffet/exported_property_set.cc
index ba6b2c7..b1b5633 100644
--- a/buffet/exported_property_set.cc
+++ b/buffet/exported_property_set.cc
@@ -51,7 +51,12 @@
   sequencer->OnAllTasksCompletedCall({cb});
 }
 
-ExportedPropertySet::~ExportedPropertySet() { }
+ExportedPropertySet::PropertyWriter ExportedPropertySet::GetPropertyWriter(
+    const std::string& interface) {
+  return base::Bind(&ExportedPropertySet::WritePropertiesDictToMessage,
+                    weak_ptr_factory_.GetWeakPtr(),
+                    interface);
+}
 
 void ExportedPropertySet::RegisterProperty(
     const std::string& interface_name,