Add pathway to auto-migrate users when Chrome launches

* For now, a command line flag is required to trigger this pathway:
  --enable-forced-migration

* UpgradeActivity kicks off the DocumentModeAssassin and waits for
  it to finish.  This Activity can only be launched via other
  ChromeActivities, and can be passed an Intent to refire when the
  migration finishes.

  * A MAIN Intent or Intent to start a new DocumentActivity that goes
    through ChromeLauncherActivity will force users through
    UgpradeActivity, instead.  This causes document mode to be flipped
    off, and anything going down the ChromeLauncherActivity pathway
    again should no longer trigger migration.

    This pathway catches new tabs created via VIEW Intents, MAIN
    Intents, CCT Intents, and (sometimes) WebappActivity Intents.

  * AsyncInitializationActivity watches for the need for the migration
    pathway to be triggered when a ChromeActivity subclass starts up.

    This pathway catches Intents that skip ChromeLauncherActivity,
    which includes ChromeTabbedActivity, DocumentActivity, CustomTabActivity,
    and WebappActivity.  These can be launched directly by selecting
    them from Android Recents, or by firing Intents to bring them
    back specifically.

  * The look of this Activity is a stand-in for when we get redlines.
    The animation is also kind of janky, but I'm aiming for functionality
    at this point.

* TabDelegate is no longer allowed to create new DocumentActivities,
  instead deferring down the ChromeLauncherActivity pathway.  This
  catches cases with push notifications triggering a tab to be
  created when they're clicked.

* The preference to enable document mode is hidden once the user
  has migrated.

BUG=582539

Review URL: https://codereview.chromium.org/1817083003

Cr-Commit-Position: refs/heads/master@{#383406}

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