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Extension background page is only allowed for the incognito profile
on sign-in screen, so in order to run the app on sign-in, the app
must be run in split incognito mode.

To avoid having two background pages per user (for original and otr
profile) in regular user session, set split incognito mode only for the
app running on signin screen (by using different manifest than for a
regular session)

TEST=
1. Make sure easy sign-in works
2. Lock screen with an incognito window open.
   Verify that the screen can be unlocked using Smart Lock
BUG=448634,445048

Review URL: https://codereview.chromium.org/834233005

Cr-Commit-Position: refs/heads/master@{#311543}