Make LevelDBObserver an associated interface to LevelDBWrapper.

content/common/leveldb_wrapper.mojom

Index: content/common/leveldb_wrapper.mojom
diff --git a/content/common/leveldb_wrapper.mojom b/content/common/leveldb_wrapper.mojom
index c929e771eda1c78a86c905a292bef417402dbf26..6295be0b9789a4075e1a7195e418557acb6022c6 100644
--- a/content/common/leveldb_wrapper.mojom
+++ b/content/common/leveldb_wrapper.mojom
@@ -30,6 +30,8 @@ struct KeyValue {
 // A wrapper around leveldb that supports giving notifications when values
 // change.
 interface LevelDBWrapper {
+  AddObserver(associated LevelDBObserver observer);
+
   // Sets the database entry for |key| to |value|. Returns OK on success.
   Put(array<uint8> key, array<uint8> value, string source) => (bool success);