WasmGraphBuilder contains fields which store state which is modified
during graph construction, other fields store information which is
never modified. Make the latter fields const.

R=mtrofin@chromium.org

Change-Id: I3ea57fbca6b24247989f5ae7260ffed2013ad82a
Reviewed-on: https://chromium-review.googlesource.com/850396
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50371}