[SyncFS] Support Delete operation on LevelDBWrapper::Iterator

chrome/browser/sync_file_system/drive_backend/leveldb_wrapper_unittest.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/sync_file_system/drive_backend/leveldb_wrapper.h"
 
 #include <string>
 
 #include "base/files/scoped_temp_dir.h"
 #include "base/logging.h"
@@ skipping 95 matching lines @@
   EXPECT_EQ("bb", itr->key().ToString());
   EXPECT_EQ("3", itr->value().ToString());
 
   itr->Next();
   EXPECT_TRUE(itr->Valid());
   EXPECT_EQ("d", itr->key().ToString());
   EXPECT_EQ("4", itr->value().ToString());
 
   itr->Next();
   EXPECT_FALSE(itr->Valid());
+
+  itr->SeekToFirst();
+  EXPECT_TRUE(itr->Valid());
+  EXPECT_EQ("a", itr->key().ToString());
+  EXPECT_EQ("1", itr->value().ToString());
+
+  itr->Delete();
+  EXPECT_TRUE(itr->Valid());
+  EXPECT_EQ("ab", itr->key().ToString());
+  EXPECT_EQ("0", itr->value().ToString());
+
+  itr->SeekToFirst();
+  EXPECT_TRUE(itr->Valid());
+  EXPECT_EQ("ab", itr->key().ToString());
+  EXPECT_EQ("0", itr->value().ToString());
 }
 
 TEST_F(LevelDBWrapperTest, Iterator2Test) {
   GetDB()->Put("a", "1");
   GetDB()->Put("b", "2");
   GetDB()->Put("c", "3");
   // Keep pending transanctions on memory.
 
   scoped_ptr<LevelDBWrapper::Iterator> itr = GetDB()->NewIterator();
 
@@ skipping 75 matching lines @@
   GetDB()->Delete("ab");
   GetDB()->Commit();
   GetDB()->Clear();
 
   SCOPED_TRACE("DeleteTest_Commit");
   CheckDBContents(merged_data, arraysize(merged_data));
 }
 
 }  // namespace drive_backend
 }  // namespace sync_file_system