The ExternalTabContainer should only invoke unload handlers on the page in its OnDestroy handler. Doing it in the destructor as well causes DCHECKs to fire in the code path which checks if it is the innermost ExternalTab for which we received a CloseContents notification and if not post the task back to be handled later. Posting  a task from the destructor causes the DCHECK as it tries to grab a ref on the object.
The ExternalTabContainer should only invoke unload handlers on the page in its OnDestroy handler. Doing it
in the destructor as well causes DCHECKs to fire in the code path which checks if it is the innermost ExternalTab
for which we received a CloseContents notification and if not post the task back to be handled later. Posting 
a task from the destructor causes the DCHECK as it tries to grab a ref on the object.

Part of the fix for http://code.google.com/p/chromium/issues/detail?id=35729

Bug=35279

Review URL: http://codereview.chromium.org/651051

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@39498 0039d316-1c4b-4281-b951-d872f2087c98

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