[Chromecast] Moves metrics client ID/providers to CastContentBrowserClient.

This removes platform_metrics_providers.h and its link-time implementations.

R=byungchul@chromium.org,gfhuang@chromium.org,halliwell@chromium.org
BUG=internal b/20121400

Review URL: https://codereview.chromium.org/1298473004

Cr-Commit-Position: refs/heads/master@{#344371}
11 files changed