This adds a new external type (v8::SharedArrayBuffer) that uses a JSArrayBuffer
under the hood. It can be distinguished from an ArrayBuffer by the newly-added
is_shared() bit.

Currently there is no difference in functionality between a SharedArrayBuffer
and an ArrayBuffer. However, a future CL will add the Atomics API, which is
only available on an SharedArrayBuffer. All non-atomic accesses are identical
to ArrayBuffer accesses.

LOG=N
BUG=

Review URL: https://codereview.chromium.org/1136553006

Cr-Commit-Position: refs/heads/master@{#28594}