metrics: Make implicit MetricsLibrary d'tor explicit.

Also add 'virtual' and 'OVERRIDE' to overridden methods.

BUG=none
TEST=did a build

Change-Id: Iec71c59853b6ada38b20cea9d0605c3d39e9353f
Reviewed-on: https://chromium-review.googlesource.com/189398
Reviewed-by: Daniel Erat <derat@chromium.org>
Commit-Queue: Daniel Erat <derat@chromium.org>
Tested-by: Daniel Erat <derat@chromium.org>
diff --git a/metrics/metrics_library.cc b/metrics/metrics_library.cc
index c53c16b..1214dd0 100644
--- a/metrics/metrics_library.cc
+++ b/metrics/metrics_library.cc
@@ -88,6 +88,8 @@
     : uma_events_file_(NULL),
       consent_file_(kConsentFile) {}
 
+MetricsLibrary::~MetricsLibrary() {}
+
 // We take buffer and buffer_size as parameters in order to simplify testing
 // of various alignments of the |device_name| with |buffer_size|.
 bool MetricsLibrary::IsDeviceMounted(const char* device_name,