UMA Metric for reporting network problem while connected.

Added a UMA metric for reporting network problem while connected to
a network. Network problem is detected by TrafficMonitor. Max of one
network problem will be reported per connection.

BUG=chromium:374274
TEST=unit tests, manual
Manual Test:
1. Connect a chrome device an AP without internet access.
2. Wait for few minutes, then browse to "chrome://histograms", and verify
   there is a histogram for Network.Shill.Wifi.NetworkProblemDetected".

Change-Id: Iad019d147feebcd429445c687a7c37d8e1e281f9
Reviewed-on: https://chromium-review.googlesource.com/200469
Reviewed-by: Peter Qiu <zqiu@chromium.org>
Commit-Queue: Peter Qiu <zqiu@chromium.org>
Tested-by: Peter Qiu <zqiu@chromium.org>