Stop TextIterator from emitting double newlines after H1 and H2

TextIterator currently may add two consecutive newlines after an
H1/2 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.

Most of this patch is rebaselining test results. Only the following
two files contains non-trivial change:
- editing/iterators/TextIterator.cpp: the implementation change
- LayoutTests/TestExpectations: found one more layout tests where
  editing command relying on incorrect plain text char count fails

Bug: 752672,758816
Change-Id: I524ef1e95c9c25372b9ba350e3d70bc5d604985e
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Reviewed-on: https://chromium-review.googlesource.com/665378
Reviewed-by: Koji Ishii <kojii@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501988}
627 files changed