Update chromecast/OWNERS.
Update chromecast/OWNERS.

Add slan@ and alokp@; remove gunsch@.

BUG=none

Review URL: https://codereview.chromium.org/1682023002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#374444}

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