Merge page serializers [8/12]

Clean up DTD test

Cleanup of PageSerializerTest.DTD test.

--

This review is part of set aiming to merge the two existing page
serializers, WebPageSerializerImpl and PageSerializer into the latter
one.

In addition to this it moves all the old tests from
WebPageNewSerializerTest and WebPageSerializerTest to the newer
PageSerializerTest structure and splits out a test for MHTML into the
MHTMLTest file.

Note: This is a rebase and split of
https://codereview.chromium.org/68613003 from Opera which was reverted
due to failing tests. In addition to the rebase some smaller fixes was
done to accommodate to changes in the code from when it was first
landed.

BUG=328354
R=hajimehoshi@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=196926

[Tiger (Sony Mobile), 2015-06-10 10:33:52]

PTAL

[hajimehoshi, 2015-06-11 00:52:53]

On 2015/06/10 10:33:52, Tiger (Sony Mobile) wrote:
> PTAL

lgtm

[philipj_slow, 2015-06-11 06:21:13]

philipj@opera.com changed reviewers:
+ philipj@opera.com

[philipj_slow, 2015-06-11 06:21:13]

lgtm

[Tiger (Sony Mobile), 2015-06-11 07:20:32]

The CQ bit was checked by gustav.tiger@sonymobile.com

[commit-bot: I haz the power, 2015-06-11 07:20:47]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1169033004/1

[commit-bot: I haz the power, 2015-06-11 08:24:45]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=196926