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This CL moves the incognito interstial instrumentation test to its own
module.

Bug: 1149867
Change-Id: If4b8bc80580c925d015e7ca896ce7eb070e74fc4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2544683
Commit-Queue: Alice Wang <aliceywang@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828654}