The new spec has two arguments, the first is the global descriptor, and
the second is the initial value:

    new WebAssembly.Global({type: i32}, 42);

If the initial value argument is omitted, the value is set to 0.

Bug: v8:7625
Change-Id: I679d4b7c49c69ec7ffcdfeb8ae506fa7ab9bba95
Reviewed-on: https://chromium-review.googlesource.com/1028847Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Ben Smith <binji@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52822}