Build a few more extensions files only when extensions are enabled.

Build a few more extensions files only when extensions are enabled.

Also move a class implementation out of feature_channel.h.

BUG=424746
Committed: https://crrev.com/f55539c55ab085f862b32ad26df420b0b0c1d7a4
Cr-Commit-Position: refs/heads/master@{#300343}

[Lei Zhang, 2014-10-17 23:41:20]

thestig@chromium.org changed reviewers:
+ yoz@chromium.org

[Yoyo Zhou, 2014-10-20 20:55:41]

LGTM

[Lei Zhang, 2014-10-20 20:57:33]

The CQ bit was checked by thestig@chromium.org

[commit-bot: I haz the power, 2014-10-20 20:58:26]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/661593005/1

[commit-bot: I haz the power, 2014-10-20 21:50:53]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-10-20 21:51:56]

Patchset 1 (id:??) landed as
https://crrev.com/f55539c55ab085f862b32ad26df420b0b0c1d7a4
Cr-Commit-Position: refs/heads/master@{#300343}