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This is similar to:
 * https://chromium-review.googlesource.com/c/chromium/src/+/1323109
 * https://chromium-review.googlesource.com/c/chromium/src/+/1329163
 * https://chromium-review.googlesource.com/c/chromium/src/+/1323070
 * https://chromium-review.googlesource.com/c/chromium/src/+/1323092

On request success, it makes the MimeSniffingURLLoader to always send a
response's body datapipe, even when it is empty.

It also adds a DCHECK to ensure the |source_url_loader| is also behaving
the same way.

Bug: 831155,826868
Change-Id: I468a24c31142cb0a8d646cd09928f7b03680d779
Reviewed-on: https://chromium-review.googlesource.com/c/1350874Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612152}