[Files.app] Reenable TraverseSlideImagesOnDownloads tests

[Files.app] Reenable TraverseSlideImagesOnDownloads tests

According to the issue (crbug.com/437293), the root issue is solved. Then we need to re-enable the disabled tests. This patch does it.

BUG=437293
TEST=browsertest passes
R=hirono@chromium.org, yawano@chromium.org

Committed: https://crrev.com/397663a2d96e717a4b33aee7f60651adb3e2a135
Cr-Commit-Position: refs/heads/master@{#320867}

[yoshiki, 2015-03-16 18:13:51]

yoshiki@chromium.org changed reviewers:
+ hirono@chromium.org

[yoshiki, 2015-03-16 18:14:17]

yoshiki@chromium.org changed reviewers:
+ yawano@chromium.org

[yoshiki, 2015-03-16 18:16:42]

@hirono, @yawano, these tests have been disabled for some reason after the root
cause is solved. It looks the tests can run without failure and we can re-enable
them. Do you remember any reason to keep them disabled?

[yoshiki, 2015-03-16 18:17:32]

FYI: I ran them 10 times locally but can't observe failure.

[hirono, 2015-03-16 23:56:39]

lgtm, thanks!

[yawano, 2015-03-17 00:57:01]

lgtm too. Thank you!

[yoshiki, 2015-03-17 04:48:38]

The CQ bit was checked by yoshiki@chromium.org

[commit-bot: I haz the power, 2015-03-17 04:48:46]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1015493002/1

[commit-bot: I haz the power, 2015-03-17 04:52:09]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-17 04:53:06]

Patchset 1 (id:??) landed as
https://crrev.com/397663a2d96e717a4b33aee7f60651adb3e2a135
Cr-Commit-Position: refs/heads/master@{#320867}