Fix bug in ALTERNATE_PROTOCOL_USAGE_MAPPING_MISSING histogram

Fix bug in ALTERNATE_PROTOCOL_USAGE_MAPPING_MISSING histogram
in which we would consider the mapping to be missing even if
the probability was such that we would not honor the advertisement.

BUG=

Committed: https://crrev.com/413cdda9719e7e8b4e9410a9b9630e684e29fc0c
Cr-Commit-Position: refs/heads/master@{#297773}

[Ryan Hamilton, 2014-10-01 23:10:30]

rch@chromium.org changed reviewers:
+ rtenneti@chromium.org

[ramant (doing other things), 2014-10-01 23:27:58]

lgtm

[Ryan Hamilton, 2014-10-02 00:34:56]

The CQ bit was checked by rch@chromium.org

[commit-bot: I haz the power, 2014-10-02 00:35:45]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/620753007/1

[commit-bot: I haz the power, 2014-10-02 02:58:33]

Committed patchset #1 (id:1) as ae9be40467056c41518bdcedec995e1d8b96abfb

[commit-bot: I haz the power, 2014-10-02 02:59:13]

Patchset 1 (id:??) landed as
https://crrev.com/413cdda9719e7e8b4e9410a9b9630e684e29fc0c
Cr-Commit-Position: refs/heads/master@{#297773}