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In https://chromiumcodereview.appspot.com/473583002/ we removed a
'shadow' element in thumbnail <iframe> to improve customization. A side
effect is that this exposes the thumbnail image, thereby alow the user
to futilely access the chrome-search://thumb/#/# image below. This CL
adds back a transparent "blocker" in front of the thumbnail to restore
old behavior. Also making 2 CSS changes to:
- Fix fake box text positioning for RTL.
- Move top-margin back from #ntp-contents to #logo, for correctness.

BUG=411005

Review URL: https://codereview.chromium.org/542033002

Cr-Commit-Position: refs/heads/master@{#293650}