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This is the same as crrev.com/c/1724754 but for android webview.
We need a didicated connector for the IO thread to create
{URLLoader,WebSocketHandshake}ThrottleProviders.

Bug: 989781
Change-Id: Ia0c883d243e8f856ede2d2747e124b80c2253dec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1760646Reviewed-by: Bo <boliu@chromium.org>
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688510}