Fix cast private API from crashing the extension on non-CrOS platforms.
Fix cast private API from crashing the extension on non-CrOS platforms.

This happened because the chrome.cast.devicesPrivate API was defined but there
was no schema available. This CL makes the schema available but keeps it
disabled if the current platform is not CrOS.

BUG=514952

Review URL: https://codereview.chromium.org/1309293003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#345226}

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