Create NGPhysicalFragment for every SPAN element for LayoutNGPaintFragments runtime feature enabled

This patch changes |ShouldCreateBoxFragment()| to return |true| for producing
box fragments for plain SPAN when |LayoutNGPaintFragments| runtime feature
enabled, to locate |NGPhysicalFragment| associated to |LayoutInlinline| for
computing bounding box and other kinds rects.

Without this patch, we need to traverse fragment tree rooted by
|LayoutBlockFlow| containing |LayoutInline|.

In legacy layout tree, we don't generate |InlineBox| for |LayoutInline| for
performance optimization and called "Culled InlineBox". To compute rects in
legacy layout tree, we traverse layout tree and line boxes in |LayoutInline|.
This code makes |LayoutInline| implementation complicated. Note: There is a
flag disabling "Culled InlineBox" as |AlwaysCreateLineBoxesForLayoutInline|.

Once LayoutNG is stable enough to have profiling, we investigate whether
generating fragment for every SPAN causes performance regression or not.


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

Bug: 789390
Change-Id: Ife87fc9ee487397891ae4515d1bcaea1afcf9c04
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Reviewed-on: https://chromium-review.googlesource.com/790177
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520865}

third_party/WebKit/LayoutTests/TestExpectations

@@ -614,6 +614,9 @@ crbug.com/635619 [ Mac ] virtual/layout_ng/fast/block/margin-collapse/block-insi
 
 # ====== LayoutNG,LayoutNGPaintFragments ======
 
+# expected is wrong
+crbug.com/789390 virtual/layout_ng_paint/fast/inline/vertical-align-text-inherit.html [ Failure ]
+
 ### Crash site: ContainerNode.cpp
 crbug.com/714962 virtual/layout_ng_paint/fast/inline/inline-with-empty-inline-children.html [ Crash Failure ]
 

third_party/WebKit/Source/core/layout/ng/inline/ng_inline_layout_algorithm.cc

@@ -37,7 +37,17 @@ namespace {
 
 inline bool ShouldCreateBoxFragment(const NGInlineItem& item,
                                     const NGInlineItemResult& item_result) {
+  // Note: When you introduce more conditions, please make sure we have
+  // box fragments for plain SPAN to avoid using culled InlineBox which
+  // requires traversing layout tree to find |InlineBox| in SPAN, e.g.
+  //   <div><span>foo</span></div>.
+  // In legacy layout tree, we don't have |InlineBox| for plain SPAN if
+  // |LayoutObject::AlwaysCreateLineBoxes()| is false.
+  // See |LayoutBlockFlow::LayoutInlineChildren()| for conditions for
+  // enabling |AlwaysCreateLineBoxes()|.
   DCHECK(item.Style());
+  if (RuntimeEnabledFeatures::LayoutNGPaintFragmentsEnabled())
+    return true;
   const ComputedStyle& style = *item.Style();
   // TODO(kojii): We might need more conditions to create box fragments.
   return style.HasBoxDecorationBackground() || style.HasOutline() ||