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Add OAuth2 authentication for speech recognition requests from the launcher, when audio-history is opted-into.

Audio history allows a user to see the speech recognition result at https://history.google.com/history/audio. This change will attach an auth (OAuth2) token to the recognition request, which is used to associate the request with the user. This only happens when the user opts-in to audio history, and only on speech recognition from the app launcher.

This change adds a helper class that handles retrieving and refreshing the auth token, and plumbs that token into the speech recognition request.

BUG=397019

Review URL: https://codereview.chromium.org/778393002

Cr-Commit-Position: refs/heads/master@{#308270}