Fix bug when transferring SharedArrayBuffer to multiple Workers. (try 2)

d8 workers: Fix transferring SharedArrayBuffer to multiple Workers. (try 2)

Note: the previous try was reverted for occasional flaky tests. This continued
after the revert, and should be fixed by
https://codereview.chromium.org/1226143003.

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

Committed: https://crrev.com/5a9722b2ab107b0b8d97f925609d9d2f2939eecc
Cr-Commit-Position: refs/heads/master@{#29658}

[Jarin, 2015-07-08 16:32:26]

lgtm

[binji, 2015-07-14 19:29:29]

The CQ bit was checked by binji@chromium.org

[binji, 2015-07-14 19:29:30]

The patchset sent to the CQ was uploaded after l-g-t-m from jarin@chromium.org
Link to the patchset: https://codereview.chromium.org/1223813008/#ps60001
(title: "more tsan fixes")

[commit-bot: I haz the power, 2015-07-14 19:29:38]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1223813008/60001

[commit-bot: I haz the power, 2015-07-14 19:56:52]

Committed patchset #4 (id:60001)

[commit-bot: I haz the power, 2015-07-14 19:57:06]

Patchset 4 (id:??) landed as
https://crrev.com/5a9722b2ab107b0b8d97f925609d9d2f2939eecc
Cr-Commit-Position: refs/heads/master@{#29658}