gn/win: Don't strip trailing = on exec_script arguments.

BUG=588799

Review URL: https://codereview.chromium.org/1721883002

Cr-Original-Commit-Position: refs/heads/master@{#376983}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: baf2794a6e1adbd276deecd16637d4829a358ed8

tools/gn/function_exec_script.cc

diff --git a/tools/gn/function_exec_script.cc b/tools/gn/function_exec_script.cc
index d5e66ae..231693b 100644
--- a/tools/gn/function_exec_script.cc
+++ b/tools/gn/function_exec_script.cc
@@ -171,6 +171,13 @@
   // Make the command line.
   const base::FilePath& python_path = build_settings->python_path();
   base::CommandLine cmdline(python_path);
+
+  // CommandLine tries to interpret arguments by default.  Passing "--" disables
+  // this for everything following the "--", so pass this as the very first
+  // thing to python.  Python ignores a -- before the .py file, and this makes
+  // CommandLine let through arguments without modifying them.
+  cmdline.AppendArg("--");
+
   cmdline.AppendArgPath(script_path);
 
   if (args.size() >= 2) {