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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}