Bug 1036299 - Show enhanced content image when the tile is unhovered [r=adw]
Place an image above the usual thumbnail to hide on hover. Have DirectoryLinksProvider keep a cache of enhanced links/images to use on history tiles.

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extra : rebase_source : e4d1ddafebf13c1723a79d889a7ca3039396bc0d
Bug 1042204 - Control the enhancement of history tiles with a pref [r=adw]
Only show enhanced history when the pref allows it. Update newtab pages when enhanced pref changes.

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extra : rebase_source : 3a7466d119df6d0bffef4a71e9142b1e21f077ed