Revert "Stop TextIterator from emit double newlines after H4~6 elements"

This reverts commit 5605a3d7.

Reason for revert: Causing failures on all platform webkit layout test external/wpt/innerText/getter.html.

see bug: crbug.com/764417

Original change's description:
> Stop TextIterator from emit double newlines after H4~6 elements
> 
> TextIterator currently may add two consecutive newlines after an
> H4, H5 or H6 element, while the spec of innerText doesn't have
> this case at all.
> 
> This patch removes the extra newline to make TextIterator more
> spec-complicant.
> 
> Note: TextIterator should eventually remove double newline emission
> also for H1~3. This patch removes it only for H4~6 to keep the
> amount of rebaselined tests small.
> 
> Bug: 752672,758816
> Change-Id: I32583c194a829f52411555125e77590e36ccf838
> Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
> Reviewed-on: https://chromium-review.googlesource.com/661937


> Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
> Reviewed-by: Koji Ishii <kojii@chromium.org>
> Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#501297}

TBR=yosin@chromium.org,kojii@chromium.org,xiaochengh@chromium.org,simonp@opera.com

Change-Id: I16487602d07a29ac95d79867ae0522dbfbe130d5
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 752672, 758816
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Reviewed-on: https://chromium-review.googlesource.com/663941


Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Commit-Queue: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501359}