Factor out slot count calculation, and expose it so it may later be
consumed when JIT-ing to the WasmCodeManager.

Bug: 
Change-Id: I21d673b2e3d7fa4a66ae0ab6303d29cf666d743c
Reviewed-on: https://chromium-review.googlesource.com/782701Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Mircea Trofin <mtrofin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49556}