gn: Reuse IsAsciiAlpha() function from base.

tools/gn/filesystem_utils.cc

 // Copyright (c) 2013 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 "tools/gn/filesystem_utils.h"
 
 #include <algorithm>
 
 #include "base/file_util.h"
 #include "base/logging.h"
@@ skipping 81 matching lines @@
 }
 
 bool DoesBeginWindowsDriveLetter(const base::StringPiece& path) {
   if (path.size() < 3)
     return false;
 
   // Check colon first, this will generally fail fastest.
   if (path[1] != ':')
     return false;
 
-  // Check drive letter
-  if (!((path[0] >= 'A' && path[0] <= 'Z') ||
-         path[0] >= 'a' && path[0] <= 'z'))
+  // Check drive letter.
+  if (!IsAsciiAlpha(path[0]))
     return false;
 
   if (!IsSlash(path[2]))
     return false;
   return true;
 }
 #endif
 
 // A wrapper around FilePath.GetComponents that works the way we need. This is
 // not super efficient since it does some O(n) transformations on the path. If
@@ skipping 637 matching lines @@
   return GetGenDirForSourceDir(target->settings(), target->label().dir());
 }
 
 SourceDir GetCurrentOutputDir(const Scope* scope) {
   return GetOutputDirForSourceDir(scope->settings(), scope->GetSourceDir());
 }
 
 SourceDir GetCurrentGenDir(const Scope* scope) {
   return GetGenDirForSourceDir(scope->settings(), scope->GetSourceDir());
 }