[FragmentItem] Avoid excessive marking line boxes dirty

This patch changes marking lines dirty only when
|HasInlineFragments|, not when |FirstTextBox|, which is
always nullptr with LayoutNG.

|DirtyLinesFromChangedChild| is supposed to do nothing for
|NGPaintFragment|, but actually marks items dirty when
|FragmentItem| is enabled.

Also code for |InlineTextBox| was moved to inside of an if
block not to run at all for LayoutNG.

Bug: 982194
Change-Id: Icd933a1c3b6ad76b9437900bf104fc526d332dce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2237491
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#776875}

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

@@ -1760,11 +1760,12 @@ void LayoutText::SetTextWithOffset(scoped_refptr<StringImpl> text,
   int delta = new_len - old_len;
   unsigned end = len ? offset + len - 1 : offset;
 
+  bool dirtied_lines = false;
+
+  if (!IsInLayoutNGInlineFormattingContext()) {
     RootInlineBox* first_root_box = nullptr;
     RootInlineBox* last_root_box = nullptr;
 
-  bool dirtied_lines = false;
-
     // Dirty all text boxes that include characters in between offset and
     // offset+len.
     for (InlineTextBox* curr : TextBoxes()) {
@@ -1780,8 +1781,8 @@ void LayoutText::SetTextWithOffset(scoped_refptr<StringImpl> text,
         RootInlineBox* root = &curr->Root();
         if (!first_root_box) {
           first_root_box = root;
-        // The affected area was in between two runs. Go ahead and mark the root
-        // box of the run after the affected area as dirty.
+          // The affected area was in between two runs. Go ahead and mark the
+          // root box of the run after the affected area as dirty.
           first_root_box->MarkDirty();
           dirtied_lines = true;
         }
@@ -1820,9 +1821,10 @@ void LayoutText::SetTextWithOffset(scoped_refptr<StringImpl> text,
       if (curr->LineBreakObj().IsEqual(this) && curr->LineBreakPos() > end)
         curr->SetLineBreakPos(clampTo<int>(curr->LineBreakPos() + delta));
     }
+  }
 
   // If the text node is empty, dirty the line where new text will be inserted.
-  if (!FirstTextBox() && Parent()) {
+  if (!HasInlineFragments() && Parent()) {
     Parent()->DirtyLinesFromChangedChild(this);
     dirtied_lines = true;
   }