Fix PreemptiveNetworkMobileActivation histogram

This was landed with logic to log the wrong histogram name. It should be
NQE.ConnectivityMonitor.NetworkChangeEvent, not ...NetworkChangeType,
which is the name of the *enum* used by the histogram.

Bug: 1079396
Change-Id: I26a0510a061c65e9ea7ddd5b23e19c99f58c4513
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2354899
Commit-Queue: Ken Rockot <rockot@google.com>
Commit-Queue: Zhongyi Shi <zhongyi@chromium.org>
Auto-Submit: Ken Rockot <rockot@google.com>
Reviewed-by: Zhongyi Shi <zhongyi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797900}

net/nqe/connectivity_monitor.cc

@@ -222,7 +222,7 @@ void ConnectivityMonitor::OnNetworkMadeDefault(
       change_type = NetworkChangeType::kEarlyActivationOfDifferentNetwork;
     }
   }
-  UMA_HISTOGRAM_ENUMERATION("NQE.ConnectivityMonitor.NetworkChangeType",
+  UMA_HISTOGRAM_ENUMERATION("NQE.ConnectivityMonitor.NetworkChangeEvent",
                             change_type);
 
   // If we had requested activation of a mobile network from the system, we