Add [stable] marker to base token mojom.

This CL has no intended behavior change.

This is a prerequisite to passing base::Token as a stable UUID for interfaces.

Bug: 1136652
Change-Id: Icab69978c7abc5e88fbb40bfb650fecf08c9e9d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2461723
Auto-Submit: Erik Chen <erikchen@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Greg Kerr <kerrnel@chromium.org>
Commit-Queue: Greg Kerr <kerrnel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815894}

mojo/public/mojom/base/token.mojom

@@ -5,6 +5,7 @@
 module mojo_base.mojom;
 
 // Corresponds to |base::Token| defined in base/token.h
+[Stable]
 struct Token {
   uint64 high;
   uint64 low;