[Linux] Observer initiating tab for destruction in hung-renderer dialog.
[Linux] Observer initiating tab for destruction in hung-renderer dialog.

The user can close the tab out from under the dialog, or the renderer
process can crash or exit while the dialog is up.  Either way, the
TabContents references becomes stale.

Additional discussion on the Mac version of this CL:
    http://codereview.chromium.org/7810011/

BUG=89909
TEST=See bug, monitor crash server.


Review URL: http://codereview.chromium.org/7874007

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@100878 0039d316-1c4b-4281-b951-d872f2087c98

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