After the v8:8689 "Split compilation in three stages" commit landed,
WasmCompilationUnit::result_ was left dangling. In builds that
noticed this (in particular certain jumbo builds), the
-Wunused-private-field warning triggered which broke the build.

Bug: v8:8689
Change-Id: Iafc56b3dc6bb53e2e8417cabce540c2fcfd3431a
Reviewed-on: https://chromium-review.googlesource.com/c/1433780
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59059}