Oilpan: PrerenderHandle::documentWasDetached should have a null check for m_prender
Oilpan: PrerenderHandle::documentWasDetached should have a null check for m_prender

In non-oilpan, it is guaranteed that documentWasDetached() is not called
for a PrerenderHandle that is canceled, because the PrerenderHandle is
promptly removed from LivecycleNotifier::m_observers when the PrerenderHandle
gets destructed.

However, in oilpan, documentWasDetached() can be called for an already-canceled
PrerenderHandle because the removal from LivecycleNotifier::m_observers
until a next GC happens and the PrerenderHandle gets destructed. Thus we need
to check if the PrerenderHandle is already detached or not in documentWasDetached().

BUG=340522

Review URL: https://codereview.chromium.org/770913003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@186395 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs