Now that the back-end migration logic is ready, we should enable the migration UI by default.
This CL does:
- Modify encryption migration's flag type from SINGLE_VALUE_TYPE to ENABLE_DISABLE_VALUE_TYPE to have a default value.
- Enable encryption migration by default on ToT.

BUG=713454
TEST=manually tested the #enable-encryption-migration flag that "Enabled" and "Default" shows migration UI and "Disabled" doesn't.

Review-Url: https://codereview.chromium.org/2830933004
Cr-Commit-Position: refs/heads/master@{#466232}