[Sync] Add ProfileSyncService::IsSyncAllowed().
This is part of an effort to clean up sync's state interface. The new
IsSyncAllowed function is the canonical place to see whether there is
anything disallowing sync from starting up. This is conceptually
distinct from missing data (such as an auth token) required to start, or
from the user having turned sync off manually via RequestStop().
This CL also renames Profile::IsSyncAccessible to Profile::IsSyncAllowed
for consistent wording.
BUG=495192
TBR=finnur,mlerman,estade
Review URL: https://codereview.chromium.org/
1173453003
Cr-Commit-Position: refs/heads/master@{#334049}