Add utility method for WebString to base::FilePath conversion

base/files/file_path.h

Index: base/files/file_path.h
diff --git a/base/files/file_path.h b/base/files/file_path.h
index 4eb712af0540cf503676e3b012487c8d5fbb4650..3234df7bfb69d0a41baa4c3cc1cfc1aeed56fc39 100644
--- a/base/files/file_path.h
+++ b/base/files/file_path.h
@@ -373,10 +373,10 @@ class BASE_EXPORT FilePath {
   // 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;