Support more configurability in GN toolchains

tools/gn/string_utils.cc

Index: tools/gn/string_utils.cc
diff --git a/tools/gn/string_utils.cc b/tools/gn/string_utils.cc
index 60e4e8c16c5cb44fc140edab69457189518fff73..2603c15a31e2a69927d39dc4cd299f024a243e88 100644
--- a/tools/gn/string_utils.cc
+++ b/tools/gn/string_utils.cc
@@ -168,3 +168,10 @@ std::string RemovePrefix(const std::string& str, const std::string& prefix) {
         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);
+  }
+}