Return unique_ptr<WebDataConsumerHandle::Reader> directly from obtainReader

WebDataConsumerHandle::obtainReader was split into obtainReader and
obtainReaderInternal just to hide WTF::OwnPtr in public/platform.
Since all WTF::OwnPtr is replaced with std::unique_ptr, there's no reason
to have them separately.

BUG=624696

Review-Url: https://codereview.chromium.org/2172183003
Cr-Commit-Position: refs/heads/master@{#408070}
33 files changed