Make PaintDocumentMarkers() not to crash

This patch changes |NGTextFragmentPainter<Cursor>::Paint()| to pass valid text
to |PaintDocumentMarkers()| to avoid hit |DCHECK()| in |NGFragmentItem::
InlinePositionForOffset()|.

Bug: 982194
Change-Id: I95ec067ef8d77ca44d48076f49404a57132c9fb0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1942590
Auto-Submit: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720326}

third_party/blink/renderer/core/paint/ng/ng_text_fragment_painter.cc

@@ -312,6 +312,10 @@ void PaintDocumentMarkers(GraphicsContext& context,
 
 }  // namespace
 
+StringView NGTextPainterCursor::CurrentText() const {
+  return CurrentItem()->Text();
+}
+
 const NGPaintFragment& NGTextPainterCursor::RootPaintFragment() const {
   if (!root_paint_fragment_)
     root_paint_fragment_ = paint_fragment_.Root();
@@ -475,7 +479,7 @@ void NGTextFragmentPainter<Cursor>::Paint(const PaintInfo& paint_info,
       ComputeMarkersToPaint(node, text_item.IsEllipsis());
   if (paint_info.phase != PaintPhase::kSelection &&
       paint_info.phase != PaintPhase::kTextClip && !is_printing) {
-    PaintDocumentMarkers(context, text_item, fragment_paint_info.text,
+    PaintDocumentMarkers(context, text_item, cursor_.CurrentText(),
                          markers_to_paint, box_rect.offset, style,
                          DocumentMarkerPaintPhase::kBackground, nullptr);
     if (have_selection) {
@@ -576,7 +580,7 @@ void NGTextFragmentPainter<Cursor>::Paint(const PaintInfo& paint_info,
 
   if (paint_info.phase != PaintPhase::kForeground)
     return;
-  PaintDocumentMarkers(context, text_item, fragment_paint_info.text,
+  PaintDocumentMarkers(context, text_item, cursor_.CurrentText(),
                        markers_to_paint, box_rect.offset, style,
                        DocumentMarkerPaintPhase::kForeground, &text_painter);
 }

third_party/blink/renderer/core/paint/ng/ng_text_fragment_painter.h

@@ -38,6 +38,7 @@ class NGTextPainterCursor {
 
   const NGPaintFragment& PaintFragment() const { return paint_fragment_; }
   const NGPhysicalTextFragment* CurrentItem() const { return &text_fragment_; }
+  StringView CurrentText() const;
   const NGPaintFragment& RootPaintFragment() const;
 
  private: