Avoid unnecessary casts in NavigationControllerImpl.

Avoid unnecessary casts in NavigationControllerImpl.

This will avoid further casts as we refactor session history for OOPIFs.

BUG=236848
TEST=No behavior change.

Committed: https://crrev.com/3da0387c8a32f82e18a289be66c4f8b46abd90bc
Cr-Commit-Position: refs/heads/master@{#317402}

[Charlie Reis, 2015-02-20 18:06:42]

creis@chromium.org changed reviewers:
+ alexmos@chromium.org

[Charlie Reis, 2015-02-20 18:06:42]

Alex: Sorry, this one is a bit mechanical, but it cleans things up quite a bit
and will make the upcoming changes cleaner as well.  Hooray for covariant return
types!

[alexmos, 2015-02-20 19:31:11]

LGTM.  Awesome cleanup!

[Charlie Reis, 2015-02-20 19:32:40]

The CQ bit was checked by creis@chromium.org

[commit-bot: I haz the power, 2015-02-20 19:33:40]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/949473002/20001

[commit-bot: I haz the power, 2015-02-20 21:12:49]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-02-20 21:13:41]

Patchset 2 (id:??) landed as
https://crrev.com/3da0387c8a32f82e18a289be66c4f8b46abd90bc
Cr-Commit-Position: refs/heads/master@{#317402}