Support more configurability in GN toolchains

tools/gn/escape.h

Index: tools/gn/escape.h
diff --git a/tools/gn/escape.h b/tools/gn/escape.h
index 6f8cf9cc3b10976186c8c6864c5e56efd193550a..838de94370919ed0658523c67357a9a71d811f9a 100644
--- a/tools/gn/escape.h
+++ b/tools/gn/escape.h
@@ -16,8 +16,16 @@ enum EscapingMode {
   // Ninja string escaping.
   ESCAPE_NINJA,
 
-  // For writing commands to ninja files.
+  // For writing commands to ninja files. This assumes the output is "one
+  // thing" like a filename, so will escape or quote spaces as necessary for
+  // both Ninja and the shell to keep that thing together.
   ESCAPE_NINJA_COMMAND,
+
+  // For writing preformatted shell commands to Ninja files. This assumes the
+  // output already has the proper quoting and may include special shell
+  // shell characters which we want to pass to the shell (like when writing
+  // tool commands). Only Ninja "$" are escaped.
+  ESCAPE_NINJA_PREFORMATTED_COMMAND,
 };
 
 enum EscapingPlatform {