UMA metric reporting for user-initiated wifi scan.

Created a historgram for reporting user-initiated network events,
and add an entry for user-initiated wifi scan.

Also changed the histograms for DHCPOptionFailure. Instead of having
one histogram per technology type, changed to one histogram with one
entry per technology type.

BUG=chromium:369545
TEST=unit tests, manual
Manual Test:
1. On a chrome device, open the network menu in chrome to
   initate wifi scan.
2. Browse to "chrome://histograms" to verify there is a histogram
   for "Network.Shill.UserInitiatedEvents", and a hit for value
   0 (WifiScan).

Change-Id: I6a34ed3906cdfd1d74f104e2670d78dec1768e3f
Reviewed-on: https://chromium-review.googlesource.com/197988
Reviewed-by: Peter Qiu <zqiu@chromium.org>
Commit-Queue: Peter Qiu <zqiu@chromium.org>
Tested-by: Peter Qiu <zqiu@chromium.org>