Append "ForDebugging" to SyncService::GetSyncTokenStatus

This method is only supposed to be used for internal/debugging purposes
(like chrome://sync-internals), and after https://crrev.com/c/1065741
that's actually the case!

Bug: 953272
Change-Id: I5baf03d4d9e7ccef25c59dd3b0f57ecc37ad265d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1581342
Commit-Queue: Jan Krcal <jkrcal@chromium.org>
Auto-Submit: Marc Treib <treib@chromium.org>
Reviewed-by: Jan Krcal <jkrcal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653538}