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It has not actually observed anything in over eight years, when it
stopped overriding TabContentsObserver::OnMessageReceived.
  https://codereview.chromium.org/7828025/diff/2005/chrome/browser/ui/find_bar/find_tab_helper.h

All it actually cares about is storing a WebContents*, which there are
simpler ways to do.

While here, remove two includes which no longer seem used.

Change-Id: Iabb87720cde91b5edf16a6039fe602cc2e6cb4ed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2224028
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Auto-Submit: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774461}