Native resources allocated by v8, as internal implementation detail,
and held by a Foreign object, must be released when the Isolate is
torn down. Example: wasm::WasmModule allocated by wasm compile, and
held throughout the lifetime of the WebAssembly.Module object.

This change:
- Extends Managed<CppType> with a mechanism for doing just that
- Separates the role of Managed<CppType> to be strictly an owner of
the lifetime of the native resource. For cases where that's not
desirable, we can polymorphically use Foregin.
- moves managed.h out of wasm, since it's not wasm-specific.

BUG=680065

Review-Url: https://codereview.chromium.org/2676513008
Cr-Commit-Position: refs/heads/master@{#43350}