Fix potential flakiness in Files.app test

Fix potential flakiness in Files.app test

This patch makes the banner & focus code robust with promise. And make sure the banner existences before test runs.

BUG=478640
TEST=run TabindexFocusDownloads/FileManagerBrowserTest.Test/0 100 times

Committed: https://crrev.com/47464b35a33c06fa664c4f3e009a2d732fda1c1e
Cr-Commit-Position: refs/heads/master@{#327255}

[yoshiki, 2015-04-28 08:17:11]

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

[yoshiki, 2015-04-28 08:17:51]

Fukino-san, PTAL. Thanks.

[fukino, 2015-04-28 09:29:37]

On 2015/04/28 08:17:51, yoshiki wrote:
> Fukino-san, PTAL. Thanks.

LGTM!

[yoshiki, 2015-04-28 09:33:00]

The CQ bit was checked by yoshiki@chromium.org

[commit-bot: I haz the power, 2015-04-28 09:33:26]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1105333003/1

[commit-bot: I haz the power, 2015-04-28 09:58:28]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-28 09:59:16]

Patchset 1 (id:??) landed as
https://crrev.com/47464b35a33c06fa664c4f3e009a2d732fda1c1e
Cr-Commit-Position: refs/heads/master@{#327255}