mojo leveldb: Address some comments that came after it was put in the CQ.

components/filesystem/lock_table.cc

 // Copyright 2016 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 "components/filesystem/lock_table.h"
 
 #include "components/filesystem/file_impl.h"
 
 namespace filesystem {
 
 LockTable::LockTable() {}
 
 LockTable::~LockTable() {}
 
 base::File::Error LockTable::LockFile(FileImpl* file) {
   DCHECK(file->IsValid());
+  DCHECK(file->path().IsAbsolute());
 
   auto it = locked_files_.find(file->path());
   if (it != locked_files_.end()) {
     // We're already locked; that's an error condition on Windows.
     return base::File::FILE_ERROR_FAILED;
   }
 
   base::File::Error lock_err = file->RawLockFile();
   if (lock_err != base::File::FILE_OK) {
     // Locking failed for some reason.
     return lock_err;
   }
 
   locked_files_.insert(file->path());
   return base::File::FILE_OK;
 }
 
 base::File::Error LockTable::UnlockFile(FileImpl* file) {
   auto it = locked_files_.find(file->path());
   if (it != locked_files_.end()) {
-    locked_files_.erase(it);
-
     base::File::Error lock_err = file->RawUnlockFile();
     if (lock_err != base::File::FILE_OK) {
       // TODO(erg): When can we fail to release a lock?
       NOTREACHED();
       return lock_err;
     }
+
+    locked_files_.erase(it);
   }
 
   return base::File::FILE_OK;
 }
 
 void LockTable::RemoveFromLockTable(const base::FilePath& path) {
   auto it = locked_files_.find(path);
   if (it != locked_files_.end())
     locked_files_.erase(it);
 }
 
 }  // namespace filesystem