Use phantom handles instead of weak handles

Phantom handles allow for freeing objects with one pass of GC. However,
this means that by the time the callback is invoked, the v8 object already
does no longer exist. To avoid accidential access to the dead object, there
are now two callbacks, where the first must only reset the handle, and the
second does the clean-up work.

R=tsepez@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1129253004

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