mojo_base: Add [Stable] to more types

Add [Stable] to:
* mojo_base.mojom.File
* mojo_base.mojom.ProcessId
* mojo_base.mojom.UnguessableToken

These are fundamental types that are unlikely to require
any non-compatible changes. Making them stable is required
for their use in stable versioned interfaces.

Change-Id: I6f21e248dc995f790024e86ae5afe312d51b584c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2521117
Auto-Submit: Krzysztof Olczyk <kolczyk@vewd.com>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825072}

mojo/public/mojom/base/file.mojom

@@ -5,6 +5,7 @@
 module mojo_base.mojom;
 
 // Corresponds to |base::File| in base/files/file.h
+[Stable]
 struct File {
   handle<platform> fd;
   bool async;

mojo/public/mojom/base/process_id.mojom

@@ -4,6 +4,7 @@
 
 module mojo_base.mojom;
 
+[Stable]
 struct ProcessId {
   // This is the storage for the pid, on windows the pid is a DWORD and the
   // value can be DWORD_MAX which maps gracefully to uint32 while on linux