It is only attached to the global object if the --harmony-sharedarraybuffer
flag is enabled, but this allows more objects to be added to the snapshot which
seems to reduce the amount of heap memory used per context.

Bug: chromium:724053
Change-Id: I5d1115a0e3ed9abf41cb3ab80d19d622cbef7b93
Reviewed-on: https://chromium-review.googlesource.com/534594Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Ben Smith <binji@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45930}