Implements CR deduplication for iOS.

Implements CR deduplication for iOS.
It is critical that the pref service outlives the update_client.

BUG=604563
Committed: https://crrev.com/696787e15730d21e3bd526dfc58923309858d3ef
Cr-Commit-Position: refs/heads/master@{#389891}

[waffles, 2016-04-19 18:01:53]

waffles@chromium.org changed reviewers:
+ rohitrao@chromium.org, sorin@chromium.org

[waffles, 2016-04-19 18:01:53]

Rohit, Sorin, please take a look at this CL to hook up the component updater on
iOS to a PrefService that it can use to keep deduplication information.

[rohitrao (ping after 24h), 2016-04-20 11:42:24]

LGTM

I don't see any deduplication logic in this CL specifically.  All we need to do
is hook up a PrefService and then everything will work?  Should there be tests
for this?

[waffles, 2016-04-20 16:57:50]

On 2016/04/20 11:42:24, rohitrao wrote:
> LGTM
> 
> I don't see any deduplication logic in this CL specifically.  All we need to
do
> is hook up a PrefService and then everything will work?  Should there be tests
> for this?

This is a follow-up CL to https://codereview.chromium.org/1861383004, which
implemented the logic and unit tests. At that time, I left iOS with a stub
implementation because I was nervous about introducing a crash-on-shutdown in
iOS, but there was pressure to make the branch cut.

In particular, I'm worried that I don't know the iOS shutdown flow very well,
and if the PrefService is destroyed while the component updater is still
running, a crash can occur. Based on my reading of
/ios/chrome/browser/application_context_impl.h, the component updater is
destroyed prior to the PrefService so I think this is safe, but I'm a bit
nervous because I don't have an iOS build/test setup and so haven't done a
manual test that this works. I'm also not sure how iOS canary works. Any advice
is appreciated.

[Sorin Jianu, 2016-04-26 20:11:58]

lgtm

Thank you!

[waffles, 2016-04-26 20:35:09]

The CQ bit was checked by waffles@chromium.org

[commit-bot: I haz the power, 2016-04-26 20:35:22]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1902443004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1902443004/1

[commit-bot: I haz the power, 2016-04-26 21:19:45]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2016-04-26 21:22:11]

Description was changed from

==========
Implements CR deduplication for iOS.
It is critical that the pref service outlives the update_client.

BUG=604563
==========

to

==========
Implements CR deduplication for iOS.
It is critical that the pref service outlives the update_client.

BUG=604563

Committed: https://crrev.com/696787e15730d21e3bd526dfc58923309858d3ef
Cr-Commit-Position: refs/heads/master@{#389891}
==========

[commit-bot: I haz the power, 2016-04-26 21:22:12]

Patchset 1 (id:??) landed as
https://crrev.com/696787e15730d21e3bd526dfc58923309858d3ef
Cr-Commit-Position: refs/heads/master@{#389891}