Fix vector indexing bug in SerializationUtilsTest.WriteReadTest

This CL fixes a vector indexing bug in SerializationUtilsTest.WriteReadTest
where the code was supposed to ensure that each MetricSample entry in the
vector returned by SerializationUtils::ReadAndTruncateMetricsFromFile is not
null, instead of just the first entry.

BUG=None
TEST=component_unittests

Review-Url: https://codereview.chromium.org/2388853002
Cr-Commit-Position: refs/heads/master@{#422628}

components/metrics/serialization/serialization_utils_unittest.cc

@@ -154,8 +154,8 @@ TEST_F(SerializationUtilsTest, WriteReadTest) {
   ScopedVector<MetricSample> vect;
   SerializationUtils::ReadAndTruncateMetricsFromFile(filename, &vect);
   ASSERT_EQ(vect.size(), size_t(5));
-  for (int i = 0; i < 5; i++) {
-    ASSERT_TRUE(vect[0] != NULL);
+  for (MetricSample* sample : vect) {
+    ASSERT_NE(nullptr, sample);
   }
   EXPECT_TRUE(hist->IsEqual(*vect[0]));
   EXPECT_TRUE(crash->IsEqual(*vect[1]));