The interpreter stores a weak reference to the WasmInstance it belongs
to. Make this a phantom reference, so the GC can get rid of the
interpreter and its associated memory already on the first GC.

Drive-by: Some renamings/refactorings.

R=ahaas@chromium.org

Bug: chromium:875647
Change-Id: Ib49f6d8aa820b904f07fabe7df78ddf6df944264
Reviewed-on: https://chromium-review.googlesource.com/1221806Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55831}