[LayoutNG] Add "self-paint" to ShowFragmentTree output

Now the tree looks like this:
 Box (block-flow children-inline) offset:0,34 size:600x72
    LineBox offset:31,0 size:568x18
      Box (children-inline)(self paint) offset:0,-1 size:367x19
        Text offset:0,1 size:337x17 start: 30 end: 83
      Text offset:377,0 size:191x17 start: 1 end: 30
    LineBox offset:24,18 size:575x18
      Box (children-inline)(self paint) offset:0,-1 size:575x19
        Text offset:0,1 size:200x17 start: 144 end: 171
        Text offset:200,1 size:375x17 start: 84 end: 144
    LineBox offset:21,36 size:578x18

Useful when debugging painting.

Bug: 635619
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: I0901841736b7bbad9534a3ecbcc187c9b968d15c
Reviewed-on: https://chromium-review.googlesource.com/892103
Commit-Queue: Aleks Totic <atotic@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532788}

third_party/WebKit/Source/core/layout/ng/ng_physical_fragment.cc

@@ -90,22 +90,28 @@ void AppendFragmentToString(const NGPhysicalFragment* fragment,
 
   bool has_content = false;
   if (fragment->IsBox()) {
+    const auto* box = ToNGPhysicalBoxFragment(fragment);
     if (flags & NGPhysicalFragment::DumpType) {
       builder->Append("Box");
       String box_type = StringForBoxType(*fragment);
+      has_content = true;
       if (!box_type.IsEmpty()) {
         builder->Append(" (");
         builder->Append(box_type);
         builder->Append(")");
       }
-      has_content = true;
+      if (flags & NGPhysicalFragment::DumpSelfPainting &&
+          box->HasSelfPaintingLayer()) {
+        if (box_type.IsEmpty())
+          builder->Append(" ");
+        builder->Append("(self paint)");
+      }
     }
     has_content =
         AppendFragmentOffsetAndSize(fragment, builder, flags, has_content);
 
     builder->Append("\n");
 
-    const auto* box = ToNGPhysicalBoxFragment(fragment);
     if (flags & NGPhysicalFragment::DumpSubtree) {
       const auto& children = box->Children();
       for (unsigned i = 0; i < children.size(); i++)

third_party/WebKit/Source/core/layout/ng/ng_physical_fragment.h

@@ -161,6 +161,7 @@ class CORE_EXPORT NGPhysicalFragment
     DumpOffset = 0x10,
     DumpSize = 0x20,
     DumpTextOffsets = 0x40,
+    DumpSelfPainting = 0x80,
     DumpAll = -1
   };
   typedef int DumpFlags;