Add incognito:split for gaia_auth manifest used in Hotrod case

GAIA auth extension use special manifest file for Hotrod Enrollment case.
This manifest also needs incognito:split to work on login screen.

BUG=453448
TEST=manual

Review URL: https://codereview.chromium.org/900473007

Cr-Commit-Position: refs/heads/master@{#314539}

chrome/browser/resources/gaia_auth/manifest_keyboard.json

@@ -27,6 +27,7 @@
   ],
   "content_security_policy": "default-src 'self'; script-src 'self'; frame-src *; style-src 'self' 'unsafe-inline'",
   "description": "GAIA Component Extension",
+  "incognito": "split",
   "web_accessible_resources": [
     "main.css",
     "main.html",