Set a zero suggest impression for most visited.

Set a zero suggest impression for most visited.

Ensures that we can figure out how often zero suggest is triggered in a
session.

BUG=

Committed: https://crrev.com/1535e6a957b41e0e5632ee050c04c86220620d1a
Cr-Commit-Position: refs/heads/master@{#321531}

[Maria, 2015-03-19 23:21:23]

mariakhomenko@chromium.org changed reviewers:
+ hfung@chromium.org

[H Fung, 2015-03-19 23:38:26]

lgtm

I'm not sure if you want to consider adding a separate histogram for mostvisited
later also.

[Maria, 2015-03-20 00:39:38]

mariakhomenko@chromium.org changed reviewers:
+ mpearson@chromium.org

[Maria, 2015-03-20 00:39:38]

+ Mark for owners

[Mark P, 2015-03-20 04:23:20]

lgtm

[Maria, 2015-03-20 06:15:54]

The CQ bit was checked by mariakhomenko@chromium.org

[commit-bot: I haz the power, 2015-03-20 06:16:10]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1002263006/1

[commit-bot: I haz the power, 2015-03-20 07:48:59]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-20 07:49:47]

Patchset 1 (id:??) landed as
https://crrev.com/1535e6a957b41e0e5632ee050c04c86220620d1a
Cr-Commit-Position: refs/heads/master@{#321531}