Restrict web-bluetooth flag to ChromeOS, MacOS and Android
Restrict web-bluetooth flag to ChromeOS, MacOS and Android

To make it clear to developers which platforms currently support the
Web Bluetooth API, let's not clutter the about:flags page with a flag
that has no effect.
We'll update supported platforms to this flag as we implement support for them.

BUG=529547

Review URL: https://codereview.chromium.org/1410823002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#354754}

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