This CL estimates the sizes of the important managed objects in WASM:
the decoded module {WasmModule}, the native module that contains code
{NativeModule}, and the natively-allocated indirect and import tables
{WasmInstanceNativeAllocations}.

Since Managed<T> updates the isolate's external allocated memory,
it is no longer necessary to do so upon committing or releasing a
native module's memory.

R=mstarzinger@chromium.org
CC=ulan@chromium.org

Bug: v8:7424
Change-Id: Iff4e07d0d328383a925febd654ccbfc95f0930e9
Reviewed-on: https://chromium-review.googlesource.com/1079067
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53675}