Add a new test case for Element#innerText in Web Platform Test

This patch introduces a new test case for Element#innerText to verify that
result has a collapsed whitespace due by soft line break.

In the test case, the layout object associated to space character between
<span> elements, doesn't have neither InlineTextBox and NGPhysicalTextFragment.
Thus, Element#innerText implementation should detect this and handle it.

Bug: 651764
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: Ia25d06a2a3fe993f2e966188b81f9e9c1d0fe982
Reviewed-on: https://chromium-review.googlesource.com/1124264
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572164}
3 files changed