[wasm] Make CompilationHelper own the WasmModule

Another fix for more explicit management of ownership. The
CompilationHelper now always owns the WasmModule, and transfers
ownership to the generated WasmModuleWrapper (a Managed<WasmModule>)
once that object is created. Since the stored uniqe_ptr cannot be
accessed any more after this transfer, the creation of the
WasmModuleWrapper is delayed until it is really needed (step 5 in async
compilation).

R=ahaas@chromium.org

Change-Id: I22dea2e14a364ddf76751d97bd0d736a4c0ceff4
Reviewed-on: https://chromium-review.googlesource.com/498507Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45164}