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1. URLLoaders can't be reused, so we need to construct one
   per-load. This became too gnarly to handle with callbacks
   so I put it back to having a loader object that owns the
   URLLoader.

2. The relevant URLLoader needs to be passed to
   ContentHandler::OnConnect() so that it stays alive long
   enough to read the entire response.

BUG=

Review URL: https://codereview.chromium.org/513573002

Cr-Commit-Position: refs/heads/master@{#292460}