Chromecast: adds callback for CastService to control metrics state.

Chromecast: adds callback for CastService to control metrics state.

This allows CastService implementers to provide the user option for
controlling whether metrics is enabled/disabled.

R=lcwu@chromium.org
BUG=400925
Committed: https://crrev.com/7b23fd179076904d083d9a0df047806bf31508a1
Cr-Commit-Position: refs/heads/master@{#297243}

[lcwu1, 2014-09-29 19:33:55]

lgtm

[gunsch, 2014-09-29 19:51:56]

The CQ bit was checked by gunsch@chromium.org

[commit-bot: I haz the power, 2014-09-29 19:52:31]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/608093002/40001

[commit-bot: I haz the power, 2014-09-29 20:06:23]

Committed patchset #3 (id:40001) as d2dc00d59e15095fa4f9fea2108a60a68eb46049

[commit-bot: I haz the power, 2014-09-29 20:07:10]

Patchset 3 (id:??) landed as
https://crrev.com/7b23fd179076904d083d9a0df047806bf31508a1
Cr-Commit-Position: refs/heads/master@{#297243}