[proguard] Add VisibleToTesting to clearHistory on NavigationController.

[proguard] Add VisibleToTesting to clearHistory on NavigationController.

Only used in tests currently, so we need to prevent it from being removed.

BUG=
NOTRY=true

Committed: https://crrev.com/11383c4feb2c81c916c7d01d1c636fa698daeb1b
Cr-Commit-Position: refs/heads/master@{#296103}

[Ted C, 2014-09-22 23:58:17]

tedchoc@chromium.org changed reviewers:
+ aurimas@chromium.org

[Ted C, 2014-09-22 23:58:18]

PTAL

[aurimas (slooooooooow), 2014-09-22 23:59:05]

lgtm

[Ted C, 2014-09-23 00:12:09]

The CQ bit was checked by tedchoc@chromium.org

[commit-bot: I haz the power, 2014-09-23 00:13:16]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/591283002/1

[commit-bot: I haz the power, 2014-09-23 00:14:09]

Committed patchset #1 (id:1) as 693c8e2dbc78b6c8a8e76ab649bed133a794657b

[commit-bot: I haz the power, 2014-09-23 00:14:54]

Patchset 1 (id:??) landed as
https://crrev.com/11383c4feb2c81c916c7d01d1c636fa698daeb1b
Cr-Commit-Position: refs/heads/master@{#296103}