Metric report for result of user-initiated wifi connection

Added an UMA metric for result of user-initiated wifi connection.

BUG=chromium:369135
TEST=unit tests, manual
Manual Test:
1. Initiate a connection to a wifi service on a chrome device.
2. Browse to "chrome://histograms" after connection is established.
3. Verify there is a historgram for
   "Network.Shill.WiFi.UserInitiatedConnectionResult", and there
   is a hit for value 0 (success).
4. Initiate a connection to a secure wifi service, and intentionally
   enter a wrong password.
5. Browse to "chrome://histograms" after connection is failed, and
   verify there is a hit for value 1 (failure).
6. Initiate a connection to a wifi service, and then initiated another
   connection to a different wifi service before the connection is
   established to the first wifi service.
7. Browse to "chrome://histograms", verify there is a hit for
   value 2 (aborted).

Conflicts:
	metrics.cc
	metrics.h
	metrics_unittest.cc
	mock_metrics.h

Change-Id: I6d7ffe9f73334715775d76d17a9b977a926c4b94
Reviewed-on: https://chromium-review.googlesource.com/197813
Reviewed-by: Peter Qiu <zqiu@chromium.org>
Commit-Queue: Peter Qiu <zqiu@chromium.org>
Tested-by: Peter Qiu <zqiu@chromium.org>