[LayoutNG] Introduce CanUseInlineBox() for tracking project progress

This patch introduces |CanUseInlineBox()| as a preparation to add
|DCHECK(CanUseInlineBox())| to functions using |InlineBox| for marker of
progress of LayoutNG project. To release LayoutNG, we should not hit
these |DCHECK()|.

Following patch will utilize |CanUseInlineBox()| into layout/ and editing/.

The patch[1] is a sample of utilization.

[1] http://crrev.com/c/763188 [LayoutNG] Utilize DCHECK(CanUseInlineBox()) in
core/layout/


Bug: 591099, 707656
Change-Id: Id71c39ae8520ab18fd1b8b8f3e18a66e8a7d567c
Reviewed-on: https://chromium-review.googlesource.com/768572Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516239}

third_party/WebKit/Source/core/layout/line/InlineBox.cpp

@@ -372,6 +372,13 @@ void InlineBox::SetLineLayoutItemShouldDoFullPaintInvalidationIfNeeded() {
     line_layout_item_.SetShouldDoFullPaintInvalidation();
 }
 
+bool CanUseInlineBox(const LayoutObject& node) {
+  DCHECK(node.IsText() || node.IsInline() || node.IsLayoutBlockFlow());
+  return !RuntimeEnabledFeatures::LayoutNGEnabled() ||
+         !RuntimeEnabledFeatures::LayoutNGPaintFragmentsEnabled() ||
+         !node.EnclosingNGBlockFlow();
+}
+
 }  // namespace blink
 
 #ifndef NDEBUG

third_party/WebKit/Source/core/layout/line/InlineBox.h

@@ -539,6 +539,10 @@ inline void InlineBox::SetHasBadParent() {
 // interchangeably.
 DEFINE_COMPARISON_OPERATORS_WITH_REFERENCES(InlineBox)
 
+// TODO(layout-dev): Once LayoutNG supports inline layout, we should remove
+// |CanUseInlineBox()|.
+bool CanUseInlineBox(const LayoutObject&);
+
 }  // namespace blink
 
 #ifndef NDEBUG