[FragmentItem] Utilize LayoutText::FirstLineBoxTopLeft() in NGInlineCursor

This patch changes |LayoutText::FirstLineBoxTopLeft()| to use |NGInlineCursor|
for prepration of migrating |NGFragmentItem|.

Bug: 982194
Change-Id: I37d5f298056f4141de824f8eee38e88cbd49721b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2021566
Commit-Queue: Koji Ishii <kojii@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Auto-Submit: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735770}

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

@@ -1624,12 +1624,14 @@ UChar32 LayoutText::LastCharacterAfterWhitespaceCollapsing() const {
 }
 
 PhysicalOffset LayoutText::FirstLineBoxTopLeft() const {
-  if (const NGPaintFragment* fragment = FirstInlineFragment()) {
+  if (IsInLayoutNGInlineFormattingContext()) {
     // TODO(kojii): Some clients call this against dirty-tree, but NG fragments
     // are not safe to read for dirty-tree. crbug.com/963103
     if (UNLIKELY(!IsFirstInlineFragmentSafe()))
       return PhysicalOffset();
-    return fragment->InlineOffsetToContainerBox();
+    NGInlineCursor cursor;
+    cursor.MoveTo(*this);
+    return cursor ? cursor.CurrentOffset() : PhysicalOffset();
   }
   if (const auto* text_box = FirstTextBox()) {
     LayoutPoint location = text_box->Location();