Prepare for making BindOnce and BindRepeating nodiscard

This helps avoid, for example, creating a callback and forgetting to
pass it to the intended function.

There is an existing case of this in NativeWinAppFetcherImpl
which this CL also fixes.

Bug: None
Change-Id: I8c78d7384113a55f9c82756f5938dce1421b505e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7509258
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Owners-Override: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1575303}
NOKEYCHECK=True
GitOrigin-RevId: dedcec803494c241c48da408a116e6c612fc653d