Improve comment for Preferences::ApplyPreferences().
Improve comment for Preferences::ApplyPreferences().

There was a bad "in" which I replaced by an "if" and at that occasion
I polished the comment a bit more.

BUG=None

Review URL: https://codereview.chromium.org/1314433013

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#347632}

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