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

Review URL: https://codereview.chromium.org/1064693004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#323990}

TheTypoMaster/chromium-crosswalk,Just-D/chromium-1,TheTypoMaster/chromium-crosswalk,Chilledheart/chromium,PeterWangIntel/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,Fireblend/chromium-crosswalk,chuan9/chromium-crosswalk,ltilve/chromium,chuan9/chromium-crosswalk,Pluto-tv/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Chilledheart/chromium,axinging/chromium-crosswalk,Chilledheart/chromium,axinging/chromium-crosswalk,hgl888/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,Fireblend/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Just-D/chromium-1,PeterWangIntel/chromium-crosswalk,ltilve/chromium,PeterWangIntel/chromium-crosswalk,axinging/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,Fireblend/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,Fireblend/chromium-crosswalk,Just-D/chromium-1,chuan9/chromium-crosswalk,Just-D/chromium-1,hgl888/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,axinging/chromium-crosswalk,Fireblend/chromium-crosswalk,hgl888/chromium-crosswalk,Chilledheart/chromium,Pluto-tv/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Fireblend/chromium-crosswalk,axinging/chromium-crosswalk,Chilledheart/chromium,Fireblend/chromium-crosswalk,chuan9/chromium-crosswalk,ltilve/chromium,hgl888/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,ltilve/chromium,hgl888/chromium-crosswalk,Pluto-tv/chromium-crosswalk,axinging/chromium-crosswalk,Fireblend/chromium-crosswalk,hgl888/chromium-crosswalk,ltilve/chromium,hgl888/chromium-crosswalk,Chilledheart/chromium,hgl888/chromium-crosswalk,hgl888/chromium-crosswalk,ltilve/chromium,Just-D/chromium-1,Fireblend/chromium-crosswalk,chuan9/chromium-crosswalk,Just-D/chromium-1,Pluto-tv/chromium-crosswalk,chuan9/chromium-crosswalk,Just-D/chromium-1,TheTypoMaster/chromium-crosswalk,Just-D/chromium-1,chuan9/chromium-crosswalk,Chilledheart/chromium,PeterWangIntel/chromium-crosswalk,Pluto-tv/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,chuan9/chromium-crosswalk,Chilledheart/chromium,Pluto-tv/chromium-crosswalk,Just-D/chromium-1,Chilledheart/chromium,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,ltilve/chromium,ltilve/chromium,chuan9/chromium-crosswalk,ltilve/chromium