[Chromecast] Android Cast bot should use GN.
[Chromecast] Android Cast bot should use GN.

I missed switching this bot in the GYP->GN transition. The Android Cast
bot should use GN, as Cast has fully transitioned to GN.

BUG=

Review URL: https://codereview.chromium.org/1871923002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#386540}

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