Sort "deps" and "public_deps" when running the "gn format" command.

tools/gn/parse_tree.h

Index: tools/gn/parse_tree.h
diff --git a/tools/gn/parse_tree.h b/tools/gn/parse_tree.h
index b134d4a9065606f3e979b112f92279933b2efa07..36c511a62ab00d60a5ca3b5e74fe68a56e717ad4 100644
--- a/tools/gn/parse_tree.h
+++ b/tools/gn/parse_tree.h
@@ -372,6 +372,7 @@ class ListNode : public ParseNode {
   const std::vector<const ParseNode*>& contents() const { return contents_; }
 
   void SortAsStringsList();
+  void SortAsDepsList();
 
   // During formatting, do we want this list to always be multliline? This is
   // used to make assignments to deps, sources, etc. always be multiline lists,
@@ -390,6 +391,9 @@ class ListNode : public ParseNode {
   std::vector<SortRange> GetSortRanges() const;
 
  private:
+  template <typename Comparator>
+  void SortList(Comparator comparator);
+
   // Tokens corresponding to the [ and ]. The end token is stored in inside an
   // custom parse node so that it can have comments hung off of it.
   Token begin_token_;