media: Stop converting Rect to RectF implicitly in media_unittests.
media: Stop converting Rect to RectF implicitly in media_unittests.

TBR=dalecurtis
BUG=342848

Review URL: https://codereview.chromium.org/1305353012

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#347771}

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