BodyStreamBuffer: Avoid calling into V8 during construction

::toImpl ca call AdjustAmountOfExternalMemory for ArrayBuffers which can
trigger a V8 GC. Avoid being in the constructor when creating a
BodyStreamBuffer in Response creation.

BUG=chromium:716364

Review-Url: https://codereview.chromium.org/2871143002
Cr-Commit-Position: refs/heads/master@{#470888}
1 file changed