gn format: Sort `public` just like we sort `sources`

Since both of them list source files.

Change-Id: I1eb2516ffb0fe1efa8208562384a620eb45903dd
Reviewed-on: https://chromium-review.googlesource.com/674877Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>
Commit-Queue: Karl Wiberg <kwiberg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#503965}

tools/gn/command_format.cc

@@ -336,7 +336,7 @@ void Printer::SortIfSourcesOrDeps(const BinaryOpNode* binop) {
        binop->op().value() == "-=") &&
       ident && list) {
     const base::StringPiece lhs = ident->value().value();
-    if (lhs == "sources")
+    if (lhs == "public" || lhs == "sources")
       const_cast<ListNode*>(list)->SortAsStringsList();
     else if (lhs == "deps" || lhs == "public_deps")
       const_cast<ListNode*>(list)->SortAsDepsList();