Create a temp download directory for WebstorePrivateApiTest

Create a temp download directory for WebstorePrivateApiTest

This test was writing to the user's downloads directory instead of a
temporary path.

BUG=372017
R=asargent@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=269607

[asanka, 2014-05-10 05:17:09]

One more :)

[asargent_no_longer_on_chrome, 2014-05-10 05:27:48]

lgtm

[asanka, 2014-05-10 07:00:54]

The CQ bit was checked by asanka@chromium.org

[commit-bot: I haz the power, 2014-05-10 07:02:04]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/asanka@chromium.org/274193008/1

[commit-bot: I haz the power, 2014-05-10 08:30:29]

FYI, CQ is re-trying this CL (attempt #1).
Please consider checking whether the failures are real,
and report flakes to chrome-troopers@google.com.
The failing builders are:
  ios_rel_device_ninja on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/ios_rel_device_ninja/...)

[asanka, 2014-05-10 14:25:14]

The CQ bit was unchecked by asanka@chromium.org

[asanka, 2014-05-10 14:25:22]

The CQ bit was checked by asanka@chromium.org

[commit-bot: I haz the power, 2014-05-10 14:26:41]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/asanka@chromium.org/274193008/1

[asanka, 2014-05-10 14:50:58]

The CQ bit was unchecked by asanka@chromium.org

[asanka, 2014-05-10 14:52:12]

The CQ bit was checked by asanka@chromium.org

[commit-bot: I haz the power, 2014-05-10 14:53:20]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/asanka@chromium.org/274193008/1

[asanka, 2014-05-10 18:57:19]

The CQ bit was unchecked by asanka@chromium.org

[asanka, 2014-05-10 18:58:05]

Committed patchset #1 manually as r269607 (presubmit successful).