Revert 71327 - Attempt at fixing crash in ProcessPendingTabs. With the curren...

Revert 71327 - Attempt at fixing crash in ProcessPendingTabs. With the current code,
during closing a window with unload handlers if a tab is disconnected then
we delay cleanup by way of PostTask with the tab being
disconnected. During the time between when the disconnect happens and
the task is run the unloader handler sets still maintain a reference
to the tab. If ProcessPendingTabs is invoked during this window and
the tab is deleted, then we can crash.

I'm fixing it by making disconnect remove from the sets immediately,
but delay the call to ProcessPendingTabs. I'm also doing a similar
thing in TabDetachedAt.

BUG=15620
TEST=none

Review URL: http://codereview.chromium.org/6203006

TBR=sky@chromium.org

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=71336