testharnessreport: Hide elements in tests by removing them from the document tree, instead of visibi

testharnessreport: Hide elements in tests by removing them from the document tree, instead of visibility:hidden.

This fixes the flakiness of web-platform-tests/webrtc/datachannel-
emptystring.html.

[1] made the test flaky because:
 - It added a chance to invoke onSendChannelOpen()
 - It prints some lines into <div id=msg>.
 - Text dump of visibility:hidden doesn't remove EOLs represented by
   <br>s.

This CL resolves this issue by removing elements completely by
body.textContent = ''.  Setting display:none instead of visibility:hidden
also works, but body.textContent = '' is much simpler.

[1] https://src.chromium.org/viewvc/blink?revision=201628&view=revision
BUG=528120
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=201897

[tkent, 2015-09-08 03:15:55]

Patchset #1 (id:1) has been deleted

[tkent, 2015-09-08 05:02:02]

tkent@chromium.org changed reviewers:
+ kojii@chromium.org

[tkent, 2015-09-08 05:02:02]

kojii@, would you review this please?
This is a follow-up of https://codereview.chromium.org/1303313006/

[kojii, 2015-09-08 05:49:40]

lgtm

[tkent, 2015-09-08 06:27:09]

The CQ bit was checked by tkent@chromium.org

[commit-bot: I haz the power, 2015-09-08 06:27:19]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1332523002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1332523002/20001

[commit-bot: I haz the power, 2015-09-08 06:31:59]

Committed patchset #1 (id:20001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201897