Fix flakiness in WebstoreProviderTest.

Fix flakiness in WebstoreProviderTest.

The |mock_server_response_| field was being set in parallel with
actually running the query, so there was a race condition if the network
request came in before the response field is set.

Now sets the |mock_server_response_| before starting the query.

BUG=246136
Committed: https://crrev.com/9fd6daec4a1cee3f12740ce48c40d647f36c983a
Cr-Commit-Position: refs/heads/master@{#329364}

[Matt Giuca, 2015-05-12 05:37:53]

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

[Matt Giuca, 2015-05-12 05:37:54]

So confident that I'm titling this CL "Fix flakiness".

(I'll still keep an eye on the flakiness dashboard.)

[xiyuan, 2015-05-12 05:43:44]

LGTM

Awesome. :)

[Matt Giuca, 2015-05-12 05:57:54]

The CQ bit was checked by mgiuca@chromium.org

[commit-bot: I haz the power, 2015-05-12 05:58:42]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1137113002/1

[commit-bot: I haz the power, 2015-05-12 06:39:28]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-12 06:40:20]

Patchset 1 (id:??) landed as
https://crrev.com/9fd6daec4a1cee3f12740ce48c40d647f36c983a
Cr-Commit-Position: refs/heads/master@{#329364}