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Originally I planned to clone the request body for main resource requests,
similar to subresource requests, but it looks unnecessary for now because
main resource request bodies don't have data pipe getter elements. They
are only created by the renderer when converting from a Blob, which doesn't
happen for navigations.

So this patch:
- Adds a DCHECK to the main resource request handling code that there are
no data pipe elements.
- Adds a WPT test for network fallback with a request body (using just strings
since we can't test uploading a file with WPT as far as a I can tell, it needs
user interaction or a special test harness flag).
- Adds a http/tests/local/ test case for file upload with network fallback.
This test file was already passing, so the failing expectation is also removed.

R=kinuko, shimazu

Bug: 778878
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: I7a5f7a72ef9ac2ca3ffbe54549739ca6bcc8d071
Reviewed-on: https://chromium-review.googlesource.com/885684
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532121}