Get rid of redundant function call from LayoutInline::DirtyLinesFromChangedChild()

This patch gets rid of redundant function call introduces by the patch[1] from
|LayoutInline::DirtyLinesFromChangedChild()| and checking runtime flag before
calling non-cheap function call |EnclosingNGBlockFlow()| to avoid speed
regression.

This patch also removes a TODO comment from |SetAncestorLineBoxDirty()| call
site since the patch[2] explains it.

[1] http://crrev.com/c/768257: [LayoutNG] LayoutInline::DirtyLinesFromChangedChild()
[2] http://crrev.com/c/773455: Add a comment to LineLayoutInlineItem::SetAncestorLineBoxDirty()

Change-Id: I7eedd7753b0c901380c55c8234cc78ae65f82574
Reviewed-on: https://chromium-review.googlesource.com/774339Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517044}

third_party/WebKit/Source/core/layout/LayoutInline.cpp

@@ -1350,12 +1350,8 @@ void LayoutInline::DirtyLinesFromChangedChild(
     MarkingBehavior marking_behavior) {
   // During layout tree construction, we can't detect whether this node is
   // in LayoutNG or not.
-  if (Parent() && EnclosingNGBlockFlow()) {
-    // TODO(layout-dev): Once we supports box fragment level dirtiness,
-    // we should mark box fragments for |child| node only.
-    SelfNeedsLayout();
-    // TODO(layout-dev): We are not sure why do we need to tell ancestors
-    // line dirtiness. See also |LineLayoutItem::SetAncestorLineBoxDirty()|.
+  if (RuntimeEnabledFeatures::LayoutNGEnabled() && Parent() &&
+      EnclosingNGBlockFlow()) {
     SetAncestorLineBoxDirty();
     return;
   }