BUG=533004

TEST=
1. Enable the #enable-password-separated-signin-flow flags in chrome://flags and restart Chrome
2. In a non-signed in profile, open the User Menu and click "Sign In to Chrome"
3. The signin flow should open in a tab modal dialog
4. After entering valid credentials, the dialog should close and the Sync Confirmation dialog should be shown

Review URL: https://codereview.chromium.org/1604043004

Cr-Commit-Position: refs/heads/master@{#373653}