We currently store the {WasmModule} (generated during decoding) in a
unique_ptr and pass ownership to the {WasmModuleObject} after
compilation.
I plan to move the {Managed<NativeModule>} from {WasmCompiledModule} to
{WasmModuleObject}, which will force us to create the
{WasmModuleObject} *before* compilation, so that the {CompilationState}
is available during compilation.

This CL prepares that refactoring by storing the {WasmModule} in a
{shared_ptr} in the {AsyncCompileJob}. Note that it will eventually be
stored in a {shared_ptr} in the {Managed} anyway.

R=titzer@chromium.org

Change-Id: Iac5e3c1067af2801e938f77a455a68807801526a
Reviewed-on: https://chromium-review.googlesource.com/1104117
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53801}