Currently chunked and streaming character streams don't have cloning support as this
requires additional changes on the Blink side.

BUG=v8:8041

Change-Id: I167b3b1cd5ae1ac4038f3715b6a679d3e65d9a85
Reviewed-on: https://chromium-review.googlesource.com/1183429Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55444}