This change implements the WebAssembly.Global object and constructor,
but none of the accessors or functions.

There is a new flag to enable this: --experimental-wasm-mut-global.

Change-Id: Ifeb270d57392d7ca0900c80c0038932c96ee8b61
Reviewed-on: https://chromium-review.googlesource.com/989296
Commit-Queue: Ben Smith <binji@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52335}