Small fix for potentially unused variables in yuv_convert_unittest.cc
Small fix for potentially unused variables in yuv_convert_unittest.cc

BUG=599602

Review URL: https://codereview.chromium.org/1853483002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#384566}

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