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When it retried uploads using Channels, it would restart the upload with
data from the middle of the file, since Channels can't be rewound.  It
now just fails those requests instead.

Also, it was impossible to set the method for uploads - the default
value (POST) would overwrite any method set by the embedder.

Adds a new set of upload tests using the embedded test server, as
uploads were basically untested before.  Chunked uploads are still not
being tested, as the test server doesn't support them yet.

BUG=424712

Review URL: https://codereview.chromium.org/640593004

Cr-Commit-Position: refs/heads/master@{#300316}