NTPSnippetsDatabase: support multiple concurrent loads

NTPSnippetsDatabase: support multiple concurrent loads
(follow-up to https://codereview.chromium.org/1987333003)

So far, there are no multiple loads (yet), but this removes a weird wrinkle
from the API and doesn't really make anything any more complex.

BUG=605870
Committed: https://crrev.com/65f9bbedd611ba2fb9b1156014518111de3abf77
Cr-Commit-Position: refs/heads/master@{#397662}

[Marc Treib, 2016-06-02 12:37:18]

The CQ bit was checked by treib@chromium.org to run a CQ dry run

[Marc Treib, 2016-06-02 12:37:28]

treib@chromium.org changed reviewers:
+ bauerb@chromium.org

[commit-bot: I haz the power, 2016-06-02 12:37:37]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/2033723002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/2033723002/1

[commit-bot: I haz the power, 2016-06-02 14:15:16]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-06-02 14:15:17]

Dry run: This issue passed the CQ dry run.

[Bernhard Bauer, 2016-06-03 08:09:00]

lgtm

[Marc Treib, 2016-06-03 08:10:04]

The CQ bit was checked by treib@chromium.org

[commit-bot: I haz the power, 2016-06-03 08:10:15]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/2033723002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/2033723002/1

[commit-bot: I haz the power, 2016-06-03 08:13:52]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2016-06-03 08:16:20]

Description was changed from

==========
NTPSnippetsDatabase: support multiple concurrent loads
(follow-up to https://codereview.chromium.org/1987333003)

So far, there are no multiple loads (yet), but this removes a weird wrinkle
from the API and doesn't really make anything any more complex.

BUG=605870
==========

to

==========
NTPSnippetsDatabase: support multiple concurrent loads
(follow-up to https://codereview.chromium.org/1987333003)

So far, there are no multiple loads (yet), but this removes a weird wrinkle
from the API and doesn't really make anything any more complex.

BUG=605870

Committed: https://crrev.com/65f9bbedd611ba2fb9b1156014518111de3abf77
Cr-Commit-Position: refs/heads/master@{#397662}
==========

[commit-bot: I haz the power, 2016-06-03 08:16:21]

Patchset 1 (id:??) landed as
https://crrev.com/65f9bbedd611ba2fb9b1156014518111de3abf77
Cr-Commit-Position: refs/heads/master@{#397662}