The mutable-globals proposal spec allows importing as Numbers or
WebAssembly.Global values, but always exports as WebAssembly.Global.

Since the value is always boxed, we can also import/export i64 values.

This CL also includes support for export of mutable globals. Since the
underlying ArrayBuffer that stores the global's value is shared between
the module and the WebAssembly.Global object, all that needs to be done
is remove the validation check.

Bug: v8:7625
Change-Id: I24d763e3bc193d229a7cc33b2f2690a473c6f2bc
Reviewed-on: https://chromium-review.googlesource.com/1018406
Commit-Queue: Ben Smith <binji@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52789}