Oilpan: MutationObserver's pre-prefinalizer should be registered
Oilpan: MutationObserver's pre-prefinalizer should be registered

MutationObserver has a pre-finalizer but the pre-finalizer is never called
because the pre-finalizer is not registered to Oilpan.
This CL adds ThreadState::current()->registerPreFinalizer to register it.

BUG=340522

Review URL: https://codereview.chromium.org/810253004

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@187729 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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