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/1124264Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572164}

third_party/WebKit/LayoutTests/external/wpt/html/dom/elements/the-innertext-idl-attribute/getter-expected.txt

 This is a testharness.js-based test.
-Found 213 tests; 135 PASS, 78 FAIL, 0 TIMEOUT, 0 NOTRUN.
+Found 214 tests; 136 PASS, 78 FAIL, 0 TIMEOUT, 0 NOTRUN.
 PASS Simplest possible test ("<div>abc")
 PASS Leading whitespace removed ("<div> abc")
 PASS Trailing whitespace removed ("<div>abc ")
@@ -37,6 +37,7 @@ PASS Whitespace collapses across element boundaries ("<div><span>abc </span> def
 PASS Whitespace collapses across element boundaries ("<div><span>abc </span><span></span> def")
 FAIL Whitespace collapses across element boundaries ("<div><span>abc </span><span style='white-space:pre'></span> def") assert_equals: expected "abc def" but got "abc  def"
 PASS Soft line breaks ignored ("<div style='width:0'>abc def")
+PASS Whitespace text node preserved ("<div style='width:0'><span>abc</span> <span>def</span>")
 FAIL ::first-line styles applied ("<div class='first-line-uppercase' style='width:0'>abc def") assert_equals: expected "ABC def" but got "abc def"
 PASS ::first-letter styles applied ("<div class='first-letter-uppercase' style='width:0'>abc def")
 PASS ::first-letter float ignored ("<div class='first-letter-float' style='width:0'>abc def")

third_party/WebKit/LayoutTests/external/wpt/html/dom/elements/the-innertext-idl-attribute/getter-tests.js

@@ -56,6 +56,7 @@ testText("<div><span>abc </span><span style='white-space:pre'></span> def", "abc
 /**** Soft line breaks ****/
 
 testText("<div style='width:0'>abc def", "abc def", "Soft line breaks ignored");
+testText("<div style='width:0'><span>abc</span> <span>def</span>", "abc def", "Whitespace text node preserved");
 
 /**** first-line/first-letter ****/
 

third_party/WebKit/LayoutTests/flag-specific/enable-blink-features=LayoutNG/external/wpt/html/dom/elements/the-innertext-idl-attribute/getter-expected.txt

 This is a testharness.js-based test.
-Found 213 tests; 140 PASS, 73 FAIL, 0 TIMEOUT, 0 NOTRUN.
+Found 214 tests; 141 PASS, 73 FAIL, 0 TIMEOUT, 0 NOTRUN.
 PASS Simplest possible test ("<div>abc")
 PASS Leading whitespace removed ("<div> abc")
 PASS Trailing whitespace removed ("<div>abc ")
@@ -37,6 +37,7 @@ PASS Whitespace collapses across element boundaries ("<div><span>abc </span> def
 PASS Whitespace collapses across element boundaries ("<div><span>abc </span><span></span> def")
 PASS Whitespace collapses across element boundaries ("<div><span>abc </span><span style='white-space:pre'></span> def")
 PASS Soft line breaks ignored ("<div style='width:0'>abc def")
+PASS Whitespace text node preserved ("<div style='width:0'><span>abc</span> <span>def</span>")
 FAIL ::first-line styles applied ("<div class='first-line-uppercase' style='width:0'>abc def") assert_equals: expected "ABC def" but got "abc def"
 PASS ::first-letter styles applied ("<div class='first-letter-uppercase' style='width:0'>abc def")
 PASS ::first-letter float ignored ("<div class='first-letter-float' style='width:0'>abc def")