metrics: add PersistentInteger unit test.

BUG=chromium:339588
TEST=P2_TEST_FILTER="metrics::*" FEATURES=test emerge-panther -v platform2
BRANCH=none

Change-Id: I78bdd8eed2da69ae6284d0f7770918d1be9f0e9f
Reviewed-on: https://chromium-review.googlesource.com/190121
Reviewed-by: Luigi Semenzato <semenzato@chromium.org>
Tested-by: Luigi Semenzato <semenzato@chromium.org>
Commit-Queue: Luigi Semenzato <semenzato@chromium.org>
diff --git a/metrics/metrics.gyp b/metrics/metrics.gyp
index 2906a64..834e52f 100644
--- a/metrics/metrics.gyp
+++ b/metrics/metrics.gyp
@@ -63,6 +63,15 @@
     ['USE_test == 1', {
       'targets': [
         {
+          'target_name': 'persistent_integer_test',
+          'type': 'executable',
+          'includes': ['../common-mk/common_test.gypi'],
+          'sources': [
+            'persistent_integer.cc',
+            'persistent_integer_test.cc',
+          ]
+        },
+        {
           'target_name': 'metrics_library_test',
           'type': 'executable',
           'dependencies': [