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This adds test cases that verify that NEL reports are actually generated
and uploaded when NEL is activated.  We test both when the NEL policy is
received through HTTPS response headers, and when they're preloaded in
the Cronet experimental options config.

Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;master.tryserver.chromium.android:android_cronet_tester
Change-Id: Id00f9095b04471a3adacc58f1fbf72b5c7848503
Reviewed-on: https://chromium-review.googlesource.com/c/1174564
Commit-Queue: Misha Efimov <mef@chromium.org>
Reviewed-by: Misha Efimov <mef@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596019}