[Media Router WebUI] Put z-index of search over search results

[Media Router WebUI] Put z-index of search over search results

This change gives the search div tag a z-index so that the mouse can
interact with it when the search results are being shown, which
currently cover the search div with padding.

BUG=615284
R=apacible@chromium.org
NOTRY=true
NOPRESUBMIT=true

Review-Url: https://codereview.chromium.org/2019543002
Cr-Commit-Position: refs/heads/master@{#396414}
(cherry picked from commit c93f3868acb563e2babd9a64e883ea9832fb471b)

[btolsch, 2016-06-01 21:29:34]

Merge to M52, no conflicts. PTAL, thanks!

[apacible, 2016-06-01 22:29:51]

lgtm

[btolsch, 2016-06-01 22:38:08]

The CQ bit was checked by btolsch@chromium.org

[commit-bot: I haz the power, 2016-06-01 22:39:03]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/2035473002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/2035473002/1

[commit-bot: I haz the power, 2016-06-01 22:40:04]

Committed patchset #1 (id:1)