[NGFragmentItem] Utilize NGInlineCursor in NGTextFragmentPainterTest.TestTextStyle

This patch changes |NGTextFragmentPainterTest.TestTextStyle| to use
|NGInlineCursor| to handle both |NGFragmentItem| and |NGPaintFragment| for
prepearation of migrating |NGFragmentItem|.

Bug: 982194
Change-Id: If78b7eadb10dcc17e65139f2c134607917b04752
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2105269Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Auto-Submit: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#751259}

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

@@ -6,6 +6,7 @@
 
 #include "testing/gmock/include/gmock/gmock.h"
 #include "testing/gtest/include/gtest/gtest.h"
+#include "third_party/blink/renderer/core/layout/ng/inline/ng_inline_cursor.h"
 #include "third_party/blink/renderer/core/layout/ng/layout_ng_block_flow.h"
 #include "third_party/blink/renderer/core/layout/ng/ng_block_node.h"
 #include "third_party/blink/renderer/core/paint/ng/ng_paint_fragment.h"
@@ -43,11 +44,10 @@ TEST_P(NGTextFragmentPainterTest, TestTextStyle) {
       DocumentUpdateReason::kTest);
   Paint(IntRect(0, 0, 640, 480));
 
-  const NGPaintFragment& root_fragment = *block_flow.PaintFragment();
-  EXPECT_EQ(1u, root_fragment.Children().size());
-  const NGPaintFragment& line_box_fragment = *root_fragment.FirstChild();
-  EXPECT_EQ(1u, line_box_fragment.Children().size());
-  const NGPaintFragment& text_fragment = *line_box_fragment.FirstChild();
+  NGInlineCursor cursor;
+  cursor.MoveTo(*block_flow.FirstChild());
+  const DisplayItemClient& text_fragment =
+      *cursor.Current().GetDisplayItemClient();
 
   EXPECT_THAT(RootPaintController().GetDisplayItemList(),
               ElementsAre(IsSameId(&ViewScrollingBackgroundClient(),