Use explicit WebString <-> string16 conversion methods in storage API files

third_party/WebKit/public/platform/FilePathConversion.h

 // 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.
 
 #ifndef FilePathConversion_h
 #define FilePathConversion_h
 
 #include "WebCommon.h"
 
 namespace base {
 class FilePath;
 }
 
 namespace blink {
 
 class WebString;
 
 BLINK_COMMON_EXPORT base::FilePath WebStringToFilePath(const WebString&);
 
+BLINK_COMMON_EXPORT WebString FilePathToWebString(const base::FilePath&);
+
 }  // namespace blink
 
 #endif  // FilePathConversion_h