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ThumbnailPageEventAdapter is unused at the moment and there is no active
development in the directory.

Remove it to reduce the number of users of WebContentsObservers and
other content/public APIs and make changes to them (a little bit)
easier.

R=dfried@chromium.org

Change-Id: Ibba50f017a1ebf3b90eacf4392ea72413a97a113
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2085311Reviewed-by: Dana Fried <dfried@chromium.org>
Commit-Queue: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747458}