metrics: add Network.Shill.Wifi.UnreliableLinkSignalStrength

metrics: add Network.Shill.Wifi.UnreliableLinkSignalStrength

Add a histogram "Network.Shill.Wifi.UnreliableLinkSignalStrength"
for sampling WiFi signal strength when the WiFi network becomes
unreliable (experiencing multiple link failures in a short period
of time).

BUG=chromium:416631
TEST=pretty_print.py, validate_format.py

Committed: https://crrev.com/29123174b12adaabdf9e895374db9013a74c8fb7
Cr-Commit-Position: refs/heads/master@{#323990}

[zqiu1, 2015-04-06 23:04:41]

zqiu@chromium.org changed reviewers:
+ isherman@chromium.org, jwd@chromium.org

[Ilya Sherman, 2015-04-06 23:22:42]

LGTM

[zqiu1, 2015-04-06 23:25:14]

The CQ bit was checked by zqiu@chromium.org

[commit-bot: I haz the power, 2015-04-06 23:25:43]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1064693004/1

[commit-bot: I haz the power, 2015-04-07 00:35:52]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-07 00:36:45]

Patchset 1 (id:??) landed as
https://crrev.com/29123174b12adaabdf9e895374db9013a74c8fb7
Cr-Commit-Position: refs/heads/master@{#323990}

[zqiu1, 2015-07-07 16:33:55]

Patchset #2 (id:20001) has been deleted