It does exactly the same as GetWireBytesStorage.
Also change the WasmCompilationUnit::ExecuteCompilation method to
receive a reference to the shared_ptr, since it does not take (shared)
ownership. This saves one ref count increment and decrement.

R=titzer@chromium.org

Bug: v8:8689
Change-Id: I03c3ec5f907c738c73e0dc16646d78ecac3f3717
Reviewed-on: https://chromium-review.googlesource.com/c/1417632
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58908}