Design doc: https://docs.google.com/document/d/1rW9-NkJFTTHx-UlfkPrxtEVr6qqajKl1PgeucigC5y0/edit?ts=5bf2d92a#heading=h.gddgp5qne7pt

Currently ResourceLoader::DidStartLoadingResponseBody() is expected to be called only when the response is downloaded to a blob. As we deprecate WebDataConsumerHandle, we need to make it work when non-blob case.