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As per the CPP style guide, variables declared constexpr or const, and whose
value is fixed for the duration of the program, are named with a leading "k"
followed by mixed case. Since objects don't have a constant value for the
duration of the program, refactor "kTestString" to "test_string".

Review URL: https://codereview.chromium.org/1649573002

Cr-Commit-Position: refs/heads/master@{#372042}