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Before this CL, MessagePort/ServiceWorker/Client can send an ArrayBuffer with
copy semantics, but it cannot send an ArrayBuffer with move semantics even if
it's specified as a transferable object.

  // Copy semantics.
  port.postMessage(arrayBuffer);

  // Move semantics. Currently, this doesn't work and shows a warning message
  // instead. See https://codereview.chromium.org/1301953002
  port.postMessage(arrayBuffer, [arrayBuffer]);

After this CL, they emulate move semantics by copy-and-neuter semantics that
sends an ArrayBuffer by copy semantics and neuters the original ArrayBuffer.

<Context of this change>

Because of Chromium's multi-process architecture, a target object can be in a
different process from a sender object. This makes it difficult to support
zero-copy transfer (move semantics) because it needs to stride over process
memory boundaries. Using shared memory is a possible solution, but that seems to
need architectual changes and introduce complexity. Therefore, as a stopgap, we
support sending a transferable ArrayBuffer with copy-and-neuter semantics.

See this document for more details:
https://docs.google.com/document/d/17o_cjtc3Gk5cDZhoUc37EyhhvBb4wMlCROJvoC2Aokg/edit#

BUG=334408, 511119

Review-Url: https://codereview.chromium.org/2414333003
Cr-Commit-Position: refs/heads/master@{#427015}