Reset singleton ZoomBubbleView::zoom_bubble_ in ::Close()

      The current implementation of ZoomBubbleView is capable of attempting to re-use a zoom bubble with a stale WebContents*.

This CL resets ZoomBubbleView::zoom_bubble_ in the ZoomBubbleView::Close() method to avoid inadvertent reuse. It also adds a DCHECK to make sure WebContents* are never mis-matched in calls to ZoomBubbleView::ShowBubble().

BUG=428561

Review URL: https://codereview.chromium.org/712993004

Cr-Commit-Position: refs/heads/master@{#303945}