Handle no render view host when notified that tab contents are swapped.
Handle no render view host when notified that tab contents are swapped.

BUG=102752
TEST=See repro instructions in bug.


Review URL: http://codereview.chromium.org/8506026

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@109504 0039d316-1c4b-4281-b951-d872f2087c98

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