Added some public methods to web::NavigationController.

Added some public methods to web::NavigationController.

methods which were previously available only in Impl class:
 - GetEntryCount
 - GetItemAtIndex
 - GetCurrentEntryIndex

BUG=None

Committed: https://crrev.com/aef82172681959089da19812ce9083dc49b66f80
Cr-Commit-Position: refs/heads/master@{#348796}

[Eugene But (OOO till 7-30), 2015-09-14 23:04:12]

eugenebut@chromium.org changed reviewers:
+ kkhorimoto@google.com

[Eugene But (OOO till 7-30), 2015-09-14 23:04:12]

Stuart asked to add them in another CL:

https://codereview.chromium.org/1322193003/diff/20001/ios/web/web_state/ui/cr...

[kkhorimoto, 2015-09-15 00:50:59]

kkhorimoto@chromium.org changed reviewers:
+ kkhorimoto@chromium.org

[kkhorimoto, 2015-09-15 00:50:59]

The CQ bit was checked by kkhorimoto@chromium.org

[kkhorimoto, 2015-09-15 00:51:00]

lgtm

[commit-bot: I haz the power, 2015-09-15 00:51:09]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1342953002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1342953002/1

[commit-bot: I haz the power, 2015-09-15 01:04:01]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-15 01:04:46]

Patchset 1 (id:??) landed as
https://crrev.com/aef82172681959089da19812ce9083dc49b66f80
Cr-Commit-Position: refs/heads/master@{#348796}

[commit-bot: I haz the power, 2015-09-23 12:40:33]

Patchset 1 (id:??) landed as
https://crrev.com/aef82172681959089da19812ce9083dc49b66f80
Cr-Commit-Position: refs/heads/master@{#348796}