MD downloads: use [placeholder] instead of <label>

MD downloads: use [placeholder] instead of <label>

Chrome supports <input placeholder="Search...">. Not all browsers do,
which is why Polymer uses a <label> and swaps it in and out over inputs.
Web UI doesn't have to do this.

In addition to simplifying the code, this fixes 2 bugs:
1) <label> not being reshown after closing search/clearing input
2) cursor showing on top of text

R=tommycli@chromium.org
BUG=425626
Committed: https://crrev.com/d8537ae00027948502245db64296ae7fd915c550
Cr-Commit-Position: refs/heads/master@{#341656}

[tommycli, 2015-08-04 00:09:28]

lgtm

[Dan Beam, 2015-08-04 00:13:34]

The CQ bit was checked by dbeam@chromium.org

[commit-bot: I haz the power, 2015-08-04 00:13:46]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1264113004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1264113004/1

[commit-bot: I haz the power, 2015-08-04 00:53:08]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-04 00:54:10]

Patchset 1 (id:??) landed as
https://crrev.com/d8537ae00027948502245db64296ae7fd915c550
Cr-Commit-Position: refs/heads/master@{#341656}