[remoting host] Add User-Agent to third-party cert validation requests.
[remoting host] Add User-Agent to third-party cert validation requests.

This sets the User-Agent to the application name (Chromoting or
Chrome Remote Desktop) and version number.

BUG=611107

Review-Url: https://codereview.chromium.org/1987863002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#394507}

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