This reduces time it takes for the compiled module to be reclaimed. It
switches the reference in question from a weak reference with finalizer
to a phantom reference, because the finalizer was only clearing the
reference by now anyways.

R=ahaas@chromium.org
BUG=chromium:824443

Change-Id: I51f0dbd487281184f82fd6c79fcf27514721b819
Reviewed-on: https://chromium-review.googlesource.com/978243
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52247}