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IDL defined ReadableStream was not extendable because it didn't use
provided |info.Holder()| as its wrapper. Instead, it creates a new
wrapper in Create functions.

This CL solves the issue by making the constructor "custom". The custom
constructor creates a blink::ReadableStream first, then associates it
to the wrapper, and then calls the appropriate Init function.

Bug: 906476
Change-Id: I91a7fec1cf4ee5e08704a944b0af852584d1679e
Reviewed-on: https://chromium-review.googlesource.com/c/1347643
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610391}