UMA metric reporting for WIFI TX bitrate.

Added UMA metric for WIFI TX bitrate, which is reported every
minute when wifi is connected.

BUG=chromium:368761
TEST=unit tests, manual
Manual Test:
1. Connect a chrome device to a wireless AP
2. Browse to "chrome://histograms", and verify there is a histogram
   for "Network.Shill.WiFi.TXBitrate"

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

Change-Id: Ib4280f2df89269f30fb988904b97d35c16569218
Reviewed-on: https://chromium-review.googlesource.com/197731
Reviewed-by: Peter Qiu <zqiu@chromium.org>
Commit-Queue: Peter Qiu <zqiu@chromium.org>
Tested-by: Peter Qiu <zqiu@chromium.org>