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This CL converts DownloadStreamClientPtr to new Mojo types.
It uses Remote, PendingReceiver, and Receiver instead of
DownloadStreamClientPtr, DownloadStreamClientRequest, and
binding.

Bug: 955171
Change-Id: I20205157b4c0400b95fe0fa6ab915dbb098c7caa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1810440Reviewed-by: Sam McNally <sammc@chromium.org>
Reviewed-by: Oksana Zhuravlova <oksamyt@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Commit-Queue: Julie Kim <jkim@igalia.com>
Cr-Commit-Position: refs/heads/master@{#699674}