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When we get an NXDOMAIN results, this will look at the last result
processed and extract the TTL from the SOA record if it exists (possilbe
TODO: look at all the results and get the shortest TTL). For NODATA
results with an rcode of NOERROR and zero answers, again record a cache
entry with the TTL of the SOA if present.

R=mgersh@chromium.org

Bug: 115051
Cq-Include-Trybots: master.tryserver.chromium.android:android_cronet_tester;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I6fe029760f4ecfd3b3a91667e95b403a9f37a228
Reviewed-on: https://chromium-review.googlesource.com/562037
Commit-Queue: Brad Lassey <lassey@chromium.org>
Reviewed-by: Miriam Gershenson <mgersh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518461}