Add AppendASCII

base/file_path.h

Index: base/file_path.h
===================================================================
--- base/file_path.h	(revision 8363)
+++ base/file_path.h	(working copy)
@@ -189,6 +189,14 @@
   FilePath Append(const StringType& component) const WARN_UNUSED_RESULT;
   FilePath Append(const FilePath& component) const WARN_UNUSED_RESULT;
 
+  // Although Windows StringType is std::wstring, since the encoding it uses for
+  // paths is well defined, it can handle ASCII path components as well.
+  // Mac uses UTF8, and since ASCII is a subset of that, it works there as well.
+  // On Linux, although it can use any 8-bit encoding for paths, we assume that
+  // ASCII is a valid subset, regardless of the encoding, since many operating
+  // system paths will always be ASCII.
+  FilePath AppendASCII(const std::string& component) const WARN_UNUSED_RESULT;
+
   // Returns true if this FilePath contains an absolute path.  On Windows, an
   // absolute path begins with either a drive letter specification followed by
   // a separator character, or with two separator characters.  On POSIX