Clear window user data when WindowImpl is deleted.
Clear window user data when WindowImpl is deleted.

A WindowImpl object is owned indirectly by a TabContents. Current practice is to delete the TabContents when the parent window recieves a WM_DESTROY message. As a result the WindowImpl object may be deleted before it recieves its own WM_DESTROY message. Clearing the window user data in the WindowImpl destructor keeps WindowImpl::WndProc from crashing if the WM_DESTROY message arrives later.

BUG=113285
TEST=none

Review URL: http://codereview.chromium.org/9361049

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@121913 0039d316-1c4b-4281-b951-d872f2087c98

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