The slots for imported functions are unused by now. Shrink the table to
only store pointers for non-imported functions (i.e. wasm functions
defined in this module).

R=mstarzinger@chromium.org

Change-Id: I6d13f889528b42beca73c860a800bde7a8e921ab
Reviewed-on: https://chromium-review.googlesource.com/1084845Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53493}