Put chrome/ counterparts of WebView and MimeHandlerView under extensions/

Put chrome/ counterparts of WebView and MimeHandlerView under extensions/
namespace like other GuestViews.

This is a cleanup only CL.

Also, this CL removes one extraneous "extensions::" namespace resolution
from ChromeExtensionOptionsGuestDelegate.

BUG=None
Test=Internal cleanup only, no visible change.

Committed: https://crrev.com/0615525b39c969fc97dc281aba780477141d942f
Cr-Commit-Position: refs/heads/master@{#300582}

[lazyboy, 2014-10-21 20:39:39]

lazyboy@chromium.org changed reviewers:
+ fsamuel@chromium.org

[Fady Samuel, 2014-10-21 21:37:25]

lgtm

[lazyboy, 2014-10-21 22:28:15]

The CQ bit was checked by lazyboy@chromium.org

[commit-bot: I haz the power, 2014-10-21 22:31:53]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/668053002/20001

[commit-bot: I haz the power, 2014-10-22 00:30:19]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2014-10-22 04:04:11]

Patchset 2 (id:??) landed as
https://crrev.com/0615525b39c969fc97dc281aba780477141d942f
Cr-Commit-Position: refs/heads/master@{#300582}