When shared memory is defined in the module bytes, and not imported/exported
underlying memory should be a SharedArrayBuffer. This was missing in the
allocate flow during instantiation. Fixed to use a SharedArrayBuffer.

BUG=v8:6532

Change-Id: Ic62ed3fd578a0e03124ee40b273e6a4ea474bba4
Reviewed-on: https://chromium-review.googlesource.com/835348Reviewed-by: Eric Holk <eholk@chromium.org>
Reviewed-by: Ben Smith <binji@chromium.org>
Commit-Queue: Deepti Gandluri <gdeepti@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50255}