Chromecast: Run content_browsertests on cast_shell trybot
Chromecast: Run content_browsertests on cast_shell trybot

This should be useful to get some test coverage for codecs that are only supported on Chromecast, but are not supported by stock Chrome/Chromium (e.g. HEVC)

BUG=none

Review URL: https://codereview.chromium.org/1193333005

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#335706}

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