Sanitize access to services in HistoryCounter.

Sanitize access to services in HistoryCounter.

HistoryService:
  - Request it with EXPLICIT_ACCESS. This is valid, because the counters only count data and do not modify them (and they're also only used in the chrome://settings/ menu which cannot be opened in the incognito mode).
  - Otherwise, the counter would (crash and) fail to count history if prefs::kSavingBrowserHistoryDisabled == true, but deletion would still work, since BrowsingDataRemover also asks for EXPLICIT_ACCESS.

ProfileSyncService:
  - Do not DCHECK if it does not exist, since it is not essential for the counter to run. If the Sync service is not present, we do not get any callbacks from it, and the counter behaves the same way as if history sync was simply disabled.
  - Otherwise, this will crash if Sync service does not exist, which is a valid circumstance (e.g. when switches::kDisableSync flag is present).

BUG=510028, 553640
Committed: https://crrev.com/e42d24457084d609a882d04799fea1a7194ebc4a
Cr-Commit-Position: refs/heads/master@{#358819}

[msramek, 2015-11-10 10:03:48]

msramek@chromium.org changed reviewers:
+ engedy@chromium.org

[msramek, 2015-11-10 10:03:48]

Hi Balázs,

can you please have a look? It turns out I was bold in my expectations that the
Sync service must be present...

Thanks!
Martin

[engedy, 2015-11-10 10:36:06]

LGTM.

[msramek, 2015-11-10 10:45:59]

msramek@chromium.org changed reviewers:
+ mkwst@chromium.org

[msramek, 2015-11-10 10:45:59]

Thanks!

Mike, can you please review as the owner?

[Mike West, 2015-11-10 10:51:05]

lgtm

[msramek, 2015-11-10 10:52:43]

Thank you! Landing this now.

[msramek, 2015-11-10 10:52:49]

The CQ bit was checked by msramek@chromium.org

[commit-bot: I haz the power, 2015-11-10 10:52:52]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1413373012/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1413373012/1

[commit-bot: I haz the power, 2015-11-10 11:24:02]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-11-10 11:24:55]

Patchset 1 (id:??) landed as
https://crrev.com/e42d24457084d609a882d04799fea1a7194ebc4a
Cr-Commit-Position: refs/heads/master@{#358819}