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Not all WebContents will be zoomed by chrome_page_zoom::Zoom(), but
those that are need to have a ZoomCOntroller. It is assumed that at
the point at which the WebContents is created it should be known if
it will be passed into chrome_page_zoom::Zoom(), so the ZoomController
is created at that point.

BUG=391708

Review URL: https://codereview.chromium.org/373733003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@282221 0039d316-1c4b-4281-b951-d872f2087c98