Added support to get refresh token from disk using username and refresh token path.
Added support to get refresh token from disk using username and refresh token path.

I used a RefreshTokenStore to find, parse, and extract the correct refresh token from the .json file. I did not add unit test support to set refresh_token_store to a test_refresh_token_store in this CL, but I plan to have it when I refactor the GetRefreshToken functionality to the Chromoting environment.

All try bots succeeded.

BUG=

Review URL: https://codereview.chromium.org/1212503003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#336249}

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