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This CL makes LoadingPredictor always use the NetworkIsolationKey for
the initial main frame URL when preconnecting. In a future CL, I'll
make it use a NetworkIsolationKey of the expected destination URL,
after redirects, instead.

BUG=987735

Change-Id: I221a6bd4175005758ebf86b55cc21bdd98b167eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1716620
Commit-Queue: Matt Menke <mmenke@chromium.org>
Reviewed-by: Alex Ilin <alexilin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682722}