Update ephemeral Guest Profile handling when policy is set.

When |ForceEphemeralProfiles| is set to false, ProfileManager cannot
overwrite the pref for ephemeral Guest profiles.
To overcome the issue, a new |IsEphemeral| function is added to the
ProfileManager which not only checks for the pref value, but also
consider profile type.

Bug: 1125474
Change-Id: Ieb22c22d4ed5ac63abe0c01096e10397eb0b6531
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2584025


Reviewed-by: Dominic Battré <battre@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Commit-Queue: Ramin Halavati <rhalavati@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840017}