[LayoutNG] Implement LayoutNG version of {First,Last}CharacterAfterWhitespaceCollapsing()

This patch implements LayoutNG version of |LayoutText::
{First,Last}CharacterAfterWhitespaceCollapsing()| with making a unit test,
|CharacterAfterWhitespaceCollapsing| to run on both legacy and ng layout tree.

This patch also changes legacy layout tree path of
|LastCharacterAfterWhitespaceCollapsing()| to use |StringView::CodepointAt()|
instead of |String::CharacterStartingAt()| to return last code point in string
instead of trail part of surrogate pair.

Bug: 906915
Change-Id: I364195c5b9366cf7ca890ba2738a4be37779a6bd
Reviewed-on: https://chromium-review.googlesource.com/c/1341776
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609652}

third_party/blink/renderer/core/layout/layout_text.cc

@@ -1564,16 +1564,26 @@ UChar32 LayoutText::FirstCharacterAfterWhitespaceCollapsing() const {
     String text = text_box->GetText();
     return text.length() ? text.CharacterStartingAt(0) : 0;
   }
-  // TODO(kojii): Support LayoutNG once we have NGInlineItem pointers.
+  if (const NGPaintFragment* paint_fragment = FirstInlineFragment()) {
+    const StringView text =
+        ToNGPhysicalTextFragment(paint_fragment->PhysicalFragment()).Text();
+    return text.length() ? text.CodepointAt(0) : 0;
+  }
   return 0;
 }
 
 UChar32 LayoutText::LastCharacterAfterWhitespaceCollapsing() const {
   if (InlineTextBox* text_box = LastTextBox()) {
     String text = text_box->GetText();
-    return text.length() ? text.CharacterStartingAt(text.length() - 1) : 0;
+    return text.length() ? StringView(text).CodepointAt(text.length() - 1) : 0;
+  }
+  if (const NGPaintFragment* paint_fragment = FirstInlineFragment()) {
+    const StringView text =
+        ToNGPhysicalTextFragment(
+            paint_fragment->LastForSameLayoutObject()->PhysicalFragment())
+            .Text();
+    return text.length() ? text.CodepointAt(text.length() - 1) : 0;
   }
-  // TODO(kojii): Support LayoutNG once we have NGInlineItem pointers.
   return 0;
 }
 

third_party/blink/renderer/core/layout/layout_text_test.cc

@@ -233,9 +233,7 @@ TEST_P(MapDOMOffsetToTextContentOffset, Basic) {
   }
 }
 
-// TODO(kojii): Include LayoutNG tests by switching to
-// ParameterizedLayoutTextTest when these functions support LayoutNG.
-TEST_F(LayoutTextTest, CharacterAfterWhitespaceCollapsing) {
+TEST_P(ParameterizedLayoutTextTest, CharacterAfterWhitespaceCollapsing) {
   SetBodyInnerHTML("a<span id=target> b </span>");
   LayoutText* layout_text = GetLayoutTextById("target");
   EXPECT_EQ(' ', layout_text->FirstCharacterAfterWhitespaceCollapsing());
@@ -272,6 +270,11 @@ TEST_F(LayoutTextTest, CharacterAfterWhitespaceCollapsing) {
   DCHECK(!layout_text->HasTextBoxes());
   EXPECT_EQ(0, layout_text->FirstCharacterAfterWhitespaceCollapsing());
   EXPECT_EQ(0, layout_text->LastCharacterAfterWhitespaceCollapsing());
+
+  SetBodyInnerHTML("<b id=target>&#x1F34C;_&#x1F34D;</b>");
+  layout_text = GetLayoutTextById("target");
+  EXPECT_EQ(0x1F34C, layout_text->FirstCharacterAfterWhitespaceCollapsing());
+  EXPECT_EQ(0x1F34D, layout_text->LastCharacterAfterWhitespaceCollapsing());
 }
 
 TEST_P(ParameterizedLayoutTextTest, CaretMinMaxOffset) {