cros: Fix external cache not updating.

cros: Fix external cache not updating.

ExtensionDownloader should be created via its factory so that all parameters
needed by the ping URL would be filled out properly.

BUG=416648
Committed: https://crrev.com/f35425b2de4b7a08b84dc0745579d6964a3c8a0b
Cr-Commit-Position: refs/heads/master@{#296477}

[xiyuan, 2014-09-24 17:44:51]

xiyuan@chromium.org changed reviewers:
+ tbarzic@chromium.org

[tbarzic, 2014-09-24 18:15:35]

lgtm

[xiyuan, 2014-09-24 18:20:22]

The CQ bit was checked by xiyuan@chromium.org

[commit-bot: I haz the power, 2014-09-24 18:22:12]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/595383002/1

[commit-bot: I haz the power, 2014-09-24 18:46:48]

Committed patchset #1 (id:1) as 48fb50bd60c8296bcc0fa57c08241b4136c6837b

[commit-bot: I haz the power, 2014-09-24 18:47:53]

Patchset 1 (id:??) landed as
https://crrev.com/f35425b2de4b7a08b84dc0745579d6964a3c8a0b
Cr-Commit-Position: refs/heads/master@{#296477}

[Dmitry Polukhin, 2014-09-25 06:36:07]

dpolukhin@chromium.org changed reviewers:
+ dpolukhin@chromium.org

[Dmitry Polukhin, 2014-09-25 06:36:08]

lgtm