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This CL modularized the incognito_interstial package with its source
files, resource file and junit tests.

Bug: 1148766
Change-Id: I5ddc9a8734ff6ee7ea39d2701fe096ac39a41816
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2540469Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Alice Wang <aliceywang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827838}