[LayoutNG] Simplify NGPaintFragment::DirtyLinesFromChangedChild

This patch simplifies DirtyLinesFromChangedChild in that:
1. Clearing FirstInlineFragment for all descendants do not
   seem necessary any longer. It turned out that this makes
   reusing line boxes harder, and not doing so doesn't fail
   any tests.
2. MarkLineBoxesDirtyFor checks PaintFragment, so checking
   here does not seem necessary. Not checking can avoid
   traversing to its containing block.

Bug: 636993
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng;luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I4d485f47731355c88a554d18843c3722f822e89a
Reviewed-on: https://chromium-review.googlesource.com/c/1288174
Commit-Queue: Koji Ishii <kojii@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600797}

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

@@ -470,24 +470,8 @@ NGPaintFragment::FragmentRange NGPaintFragment::InlineFragmentsFor(
 }
 
 void NGPaintFragment::DirtyLinesFromChangedChild(LayoutObject* child) {
-  if (child->IsInline()) {
-    LayoutBlockFlow* const block = child->ContainingNGBlockFlow();
-    if (block && block->PaintFragment())
-      MarkLineBoxesDirtyFor(*child);
-  }
-  if (!child->IsInLayoutNGInlineFormattingContext())
-    return;
-  // We should rest first inline fragment for following tests:
-  //  * fast/dom/HTMLObjectElement/fallback-content-behaviour.html
-  //  * fast/dom/shadow/exposed-object-within-shadow.html
-  //  * fast/lists/inline-before-content-after-list-marker.html
-  //  * fast/lists/list-with-image-display-changed.html
-  for (LayoutObject* runner = child; runner;
-       runner = runner->NextInPreOrder(child)) {
-    if (!runner->IsInLayoutNGInlineFormattingContext())
-      continue;
-    runner->SetFirstInlineFragment(nullptr);
-  }
+  if (child->IsInline())
+    MarkLineBoxesDirtyFor(*child);
 }
 
 bool NGPaintFragment::FlippedLocalVisualRectFor(