DCHECK that predictor always has a non-empty NetworkIsolationKey.

And do the same for calls to NetworkContext::PreconnectSockets when
require_network_isolation_key_ is true.

All consumers should be passing in a non-empty NIK, even when it's
disabled at the network stack layer.

Also fix SearchEnginePreconnector bug that was causing it to pass in
empty NetworkIsolationKeys (and make its creation of a NIK a little
more efficient, while I'm at it).

This does not add a DCHECK to NetworkContext for DNS resolutions, which
will be done in another CL, since it has more consumers.

Bug: 1082280
Change-Id: I370519a9083c15b55e43badae3ad11e56c0c8aa9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3067698


Reviewed-by: Shivani Sharma <shivanisha@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Reviewed-by: Sophie Chang <sophiechang@chromium.org>
Commit-Queue: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#908696}