Fix possible loss of data warnings in media_unittests
Fix possible loss of data warnings in media_unittests

Fix compiler warning about size_t to uint8_t conversion.

BUG=

Review URL: https://codereview.chromium.org/1559013002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#367491}

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