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This fixes https://crbug.com/872284 and adds a regression test.

The bug can be triggered when the PDFIframeNavigationThrottle cancels the
PDF download. There was some code in ResourceLoader made to ignore the
cancellation. It causes the net socket to never been released. Only 6
network socket can be used on a website, so there was some kind of
resource starvation.

Bug: 872284
Change-Id: I9fcaf8da409da38e0cb4b2e9c6ad57bb2fab911c
Reviewed-on: https://chromium-review.googlesource.com/c/1348036
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Camille Lamy <clamy@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612172}