Make DownloadController singleton instance thread safe.

Make DownloadController singleton instance thread safe.

As the cost of creating download controller instance is not
too heavy its better to switch to eager initialization which
creates a ready to use instance and ensures thread safety

This method has a number of advantages:
* The instance is not constructed until the class is used.
* There is no need to synchronize the getInstance() method,
  meaning all threads will see the same instance and no (expensive)
  locking is required.
* The final keyword means that the instance cannot be redefined,
  ensuring that one (and only one) instance ever exists.
(source: http://en.wikipedia.org/wiki/Singleton_pattern#Eager_initialization)

BUG=None.

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=283430

[wajahat, 2014-07-14 12:30:17]

PTAL!

[jdduke (slow), 2014-07-14 15:01:06]

I'm not a good reviewer for this (and I'm only an owner for input-related
changes in this folder).  In the future, it should be sufficient to add a
minimal subset of owners to review your patch, unless there are additional
reviewers you'd like to focus on a particular portion of the changes.

[Yaron, 2014-07-14 17:01:44]

lgtm

[wajahat, 2014-07-15 10:33:14]

The CQ bit was checked by wajahat.s@samsung.com

[commit-bot: I haz the power, 2014-07-15 10:35:58]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/wajahat.s@samsung.com/387333002/1

[Sergiy Byelozyorov, 2014-07-16 08:51:39]

The CQ bit was unchecked by sergiyb@chromium.org

[Sergiy Byelozyorov, 2014-07-16 08:51:43]

The CQ bit was checked by sergiyb@chromium.org

[commit-bot: I haz the power, 2014-07-16 08:52:10]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/wajahat.s@samsung.com/387333002/1

[wajahat, 2014-07-16 12:52:16]

On 2014/07/16 08:51:43, Sergiy Byelozyorov wrote:
> The CQ bit was checked by mailto:sergiyb@chromium.org

@Dear Sergiyb, 
could you explain why this is taking so much time to land
as per my understanding its a very minimal patch and it should land in no time.
sorry to bother as i am getting used to trybots.
thanks for your interest in this patch :)

[Sergiy Byelozyorov, 2014-07-16 15:30:33]

On 2014/07/16 12:52:16, wajahat wrote:
> On 2014/07/16 08:51:43, Sergiy Byelozyorov wrote:
> > The CQ bit was checked by mailto:sergiyb@chromium.org
> 
> @Dear Sergiyb, 
> could you explain why this is taking so much time to land
> as per my understanding its a very minimal patch and it should land in no
time.
> sorry to bother as i am getting used to trybots.
> thanks for your interest in this patch :)

We've had a tryserver outage yesterday, so no patches were landed for over 20
hours. The master is still processing the huge backlog that was collected in
this time, which is why your CL is being delayed. Sorry for that. If you need
this urgently landed, feel free to dcommit.

[Sergiy Byelozyorov, 2014-07-16 15:32:49]

Also it appears the tree is closed, so CQ can't commit your change. I'll talk to
the sheriff.

[commit-bot: I haz the power, 2014-07-16 18:03:17]

Change committed as 283430

[wajahat, 2014-07-17 05:32:55]

On 2014/07/16 15:32:49, Sergiy Byelozyorov wrote:
> Also it appears the tree is closed, so CQ can't commit your change. I'll talk
to
> the sheriff.

Thanks Sergiy!