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Previously the local NTP thumbnails were stretched. We now make preserve
ratio preserved by setting height to "auto", but keep min-height to 100%
so if thumbnail is too short we won't see blank space.
To accommotate Material Design thumbnails, we make stored thumbnails
taller. In the thumbnail <iframe>, the extra height is hidden from view.

BUG=414878

Review URL: https://codereview.chromium.org/564973003

Cr-Commit-Position: refs/heads/master@{#295155}