Add utility method for WebString to base::FilePath conversion

base/files/file_path.h

 // Copyright (c) 2012 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.
 
 // FilePath is a container for pathnames stored in a platform's native string
 // type, providing containers for manipulation in according with the
 // platform's conventions for pathnames.  It supports the following path
 // types:
 //
 //                   POSIX            Windows
@@ skipping 355 matching lines @@
   string16 AsUTF16Unsafe() const;
 
   // Returns a FilePath object from a path name in UTF-8. This function
   // should only be used for cases where you are sure that the input
   // string is UTF-8.
   //
   // Like AsUTF8Unsafe(), this function is unsafe. This function
   // internally calls SysWideToNativeMB() on POSIX systems other than Mac
   // and Chrome OS, to mitigate the encoding issue. See the comment at
   // AsUTF8Unsafe() for details.
-  static FilePath FromUTF8Unsafe(const std::string& utf8);
+  static FilePath FromUTF8Unsafe(StringPiece utf8);
 
   // Similar to FromUTF8Unsafe, but accepts UTF-16 instead.
-  static FilePath FromUTF16Unsafe(const string16& utf16);
+  static FilePath FromUTF16Unsafe(StringPiece16 utf16);
 
   void GetSizeForPickle(PickleSizer* sizer) const;
   void WriteToPickle(Pickle* pickle) const;
   bool ReadFromPickle(PickleIterator* iter);
 
   // Normalize all path separators to backslash on Windows
   // (if FILE_PATH_USES_WIN_SEPARATORS is true), or do nothing on POSIX systems.
   FilePath NormalizePathSeparators() const;
 
   // Normalize all path separattors to given type on Windows
@@ skipping 80 matching lines @@
 template<>
 struct hash<base::FilePath> {
   size_t operator()(const base::FilePath& f) const {
     return hash<base::FilePath::StringType>()(f.value());
   }
 };
 
 }  // namespace BASE_HASH_NAMESPACE
 
 #endif  // BASE_FILES_FILE_PATH_H_