Nit: fix comment in content/public/browser/web_contents_user_data.h
Nit: fix comment in content/public/browser/web_contents_user_data.h

It was decided in https://groups.google.com/a/chromium.org/d/topic/chromium-dev/VTNZzizN0zo/discussion to mark derived class destructors as override rather than virtual.

BUG=none

Review URL: https://codereview.chromium.org/1577343002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#368949}

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