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Switch to implicit registration of prefinalizers along with removing
the ability to dynamically unregister a prefinalizer; the latter
being an unused feature.

The requirement to manually register a prefinalizer has proven to be
a chore and a source of bugs. Case in point: HTMLCanvasElement
currently declares a prefinalizer, but doesn't register it. Simplify
the programming model by automatically registering prefinalizers.

R=haraken
BUG=673645

Review-Url: https://codereview.chromium.org/2565983002
Cr-Commit-Position: refs/heads/master@{#438110}