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

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}
(cherry picked from commit d1b6f4d89d002808689a156b739f8d7fd80104fb)

TBR=wjmaclean

[wjmaclean, 2014-11-25 13:53:22]

Merged manually to branch 2214.