[wasm] Add a Managed<T> wrapper class for allocating C++ classes that are deleted when the wrapper is garbage collected.

Use sparingly!

This doesn't add any really new functionality, other than making it more
convenient to do this.

This will primarily be used to wrap a WasmModule to be referenced from a
JSObject that represents an instance. There is one WasmModule C++ object
per parsed WasmModule, so this should not be more than a handful or a few
dozen in well-behaved programs.

R=rossberg@chromium.org,mlippautz@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2409173005
Cr-Commit-Position: refs/heads/master@{#40346}