Fix bug when transferring SharedArrayBuffer to multiple Workers.

Fix bug when transferring SharedArrayBuffer to multiple Workers.

Previously, the serialization code would call Externalize for every transferred
ArrayBuffer or SharedArrayBuffer, but that function can only be called once. If
the buffer is already externalized, we should call GetContents instead.

Also fix use-after-free bug when transferring ArrayBuffers. The transferred
ArrayBuffer must be internalized in the new isolate, or be managed by the
Shell. The current code gives it to the isolate externalized and frees it
immediately afterward when the SerializationData object is destroyed.

BUG=chromium:497295
R=jarin@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1215233004

Cr-Commit-Position: refs/heads/master@{#29499}