Fix crash when swapping in a prerendered page for a page which has an unload handler. 
Fix crash when swapping in a prerendered page for a page which has an unload handler. 

***
Note: this is the fix without a browser test, for expediency. A CL with the test is in http://codereview.chromium.org/7046053/
***

The root cause PrerenderManager::OnCloseTabContentsDeleter was never unregistering itself as a TabContentsDelegate in CloseContents. 

r87606 exposed this by changing the TabContents destructor to deference the delegate if it's non-NULL. 

BUG=85350
TEST=None

Review URL: http://codereview.chromium.org/7108032

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@88478 0039d316-1c4b-4281-b951-d872f2087c98

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