Fix three crashes in the recent tabs dialog.

Fix three crashes in the recent tabs dialog.

This fixes a crash in the recent tabs dialog when clicking on a recently
closed tab and clicking the dialog's "X" button almost simultaneously.

The crash happened because when "X" is clicked, the dialog would be
dismissed and the RecentTabsPage would be destroyed. Soon thereafter,
the click on the recently closed item would be processed and it would
try to use the RecentTabsPage object.

This CL fixes the crash by tracking whether the RecentTabsManager has
been destroyed and ignoring clicks that happen after destroy has been
called. This also fixes an identical crash with clicking a foreign
session tab, and a related (but rarer) crash where sync or sign-in
related events were being processed after the RecentTabsManager was
destroyed.

BUG=567891
Review URL: https://codereview.chromium.org/1521553004

Cr-Commit-Position: refs/heads/master@{#365075}
(cherry picked from commit 7f87dbd5a96f0e29039285544428b5298cb22aa2)

[newt (away), 2015-12-14 22:18:15]

Committed patchset #1 (id:1) to pending queue manually as
181d86d0cbe696e58ca99c8d427c4e1c045205fc.