Reland "Clear user session flags before restarting Chrome for guest mode"

Reland "Clear user session flags before restarting Chrome for guest mode"

Apparently using SetFlagsForUser clears testing flags set by telemetry.
Setting the flags only if there actually are some user/policy flags should
avoid the issue.

Original description:
>
> Clear user session flags before restarting Chrome for guest mode
>
> This avoids early restart to apply guest profile user flags
>  if device owner has some flags set.
>
>  BUG=411049
>  TEST=
>  1. Login as the device owner and set some flags in about::flags
>  2. Log out and enter guest mode
>  3.1. Press alt + shift + I -> Feedback report dialog should be launched
>  3.2. In chrome://help click "Report an issue" -> Feedback report dialog
>  should be launched
>
>  Review URL: https://codereview.chromium.org/611383007
>
>  Cr-Commit-Position: refs/heads/master@{#297920}
>

Review URL: https://codereview.chromium.org/632073002

Cr-Commit-Position: refs/heads/master@{#298944}