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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}