metrics: Use the new serialization mechanism in libmetrics

Convert libmetrics to the new serialization mechanism living in
//components/metrics in the chrome repo.

BUG=chromium:374368
TEST=FEATURES=test emerge-amd64-generic metrics

Change-Id: Ia8cf128d04fedd9672fb47096dc6fd87d6a9043d
Reviewed-on: https://chromium-review.googlesource.com/207237
Reviewed-by: Luigi Semenzato <semenzato@chromium.org>
Commit-Queue: Bertrand Simonnet <bsimonnet@chromium.org>
Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>
diff --git a/metrics/libmetrics.gypi b/metrics/libmetrics.gypi
index 1753830..65de6f5 100644
--- a/metrics/libmetrics.gypi
+++ b/metrics/libmetrics.gypi
@@ -24,7 +24,10 @@
         'c_metrics_library.cc',
         'metrics_library.cc',
         'timer.cc',
+        'components/metrics/chromeos/metric_sample.cc',
+        'components/metrics/chromeos/serialization_utils.cc',
       ],
+      'include_dirs': ['.'],
     },
   ],
 }