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Fixes the recent regression with tap gesture recognition in Reader
mode tabs.  Converting a tab to show the simplified view is one of
the rare times when we get a new CVC or WebContents for the same tab.
With this change we check if the tab is connected to a different
WebContents and create a new observer when it is.

BUG=821676

Change-Id: I8ba56ffba42a6ca9c138c2cd1c387e398cc27591
Reviewed-on: https://chromium-review.googlesource.com/962992Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Donn Denman <donnd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543255}