Refactor the interface for the CrxDownloader to take a callback.

Refactor the interface for the CrxDownloader to take a callback.

To be consistent with the rest of the artifacts in the component updater,
the non-blocking calls for the CrxDownloader take a callback to be called
when the non-blocking call completes.

This change allows adding further callbacks besides the completion
callback, such as a callback to be called when download progress is made.

This change is a pure refactoring and it should be a NOP in terms of
existing functionality.

BUG=366388
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=266304

[waffles, 2014-04-24 23:27:53]

lgtm

https://codereview.chromium.org/250383008/diff/1/chrome/browser/component_upd...
File chrome/browser/component_updater/test/crx_downloader_unittest.cc (left):

https://codereview.chromium.org/250383008/diff/1/chrome/browser/component_upd...
chrome/browser/component_updater/test/crx_downloader_unittest.cc:323: 
I'm glad you got rid of this. Now if only we could get rid of it in all the
other files...

[Sorin Jianu, 2014-04-25 17:03:07]

The CQ bit was checked by sorin@chromium.org

[Sorin Jianu, 2014-04-25 21:10:01]

The CQ bit was unchecked by sorin@chromium.org

[Sorin Jianu, 2014-04-25 21:10:09]

The CQ bit was checked by sorin@chromium.org

[commit-bot: I haz the power, 2014-04-25 22:01:51]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/sorin@chromium.org/250383008/1

[commit-bot: I haz the power, 2014-04-26 00:38:58]

Change committed as 266304