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This CL ensures the ServiceWorker of the NTP gets recognized as such,
and that it is assigned the same effective URL as the NTP. This allows
it to run in the same process as the NTP when PlzNavigate is enabled,
fixing a regression on the PLT of the NTP.

BUG=705318

Review-Url: https://codereview.chromium.org/2898313003
Cr-Commit-Position: refs/heads/master@{#475566}