shill: Add Network.Shill.Cellular.TimeToEnable metrics.

This metric tracks the amount of time it takes the modem manager to
enable the modem.

BUG=chromium-os:38086
TEST=Unit tests, check chrome://histograms for new metrics

Change-Id: Ie12e8cee20586f4ebf0070721c6327c13c175474
Reviewed-on: https://gerrit.chromium.org/gerrit/41878
Reviewed-by: Arman Uguray <armansito@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>
Commit-Queue: Thieu Le <thieule@chromium.org>
Tested-by: Thieu Le <thieule@chromium.org>
diff --git a/cellular_capability_gsm_unittest.cc b/cellular_capability_gsm_unittest.cc
index a4e2914..df3b06a 100644
--- a/cellular_capability_gsm_unittest.cc
+++ b/cellular_capability_gsm_unittest.cc
@@ -77,7 +77,10 @@
                                "",
                                NULL,
                                NULL,
-                               &proxy_factory_)) {}
+                               &proxy_factory_)) {
+    metrics_.RegisterDevice(cellular_->interface_index(),
+                            Technology::kCellular);
+  }
 
   virtual ~CellularCapabilityGSMTest() {
     cellular_->service_ = NULL;