Support more configurability in GN toolchains

tools/gn/string_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/string_utils.h"
 
 #include "tools/gn/err.h"
 #include "tools/gn/scope.h"
 #include "tools/gn/token.h"
 #include "tools/gn/tokenizer.h"
@@ skipping 150 matching lines @@
     }
   }
   return true;
 }
 
 std::string RemovePrefix(const std::string& str, const std::string& prefix) {
   CHECK(str.size() >= prefix.size() &&
         str.compare(0, prefix.size(), prefix) == 0);
   return str.substr(prefix.size());
 }
+
+void TrimTrailingSlash(std::string* str) {
+  if (!str->empty()) {
+    DCHECK((*str)[str->size() - 1] == '/');
+    str->resize(str->size() - 1);
+  }
+}