[layoutng] Add some more detail to DCHECK failures

To help with debugging

R=xiaochengh@chromium.org,ikilpatrick@chromium.org

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: Ie56254eef3aaa3d21e0f74d2323211197ecd127a
Reviewed-on: https://chromium-review.googlesource.com/1013616Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Christian Biesinger <cbiesinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551229}

third_party/blink/renderer/core/layout/ng/inline/ng_caret_rect.cc

@@ -332,7 +332,7 @@ NGCaretPosition ComputeNGCaretPosition(const LayoutBlockFlow& context,
                                        unsigned offset,
                                        TextAffinity affinity) {
   const NGPaintFragment* root_fragment = context.PaintFragment();
-  DCHECK(root_fragment);
+  DCHECK(root_fragment) << "no paint fragment on layout object " << &context;
 
   NGCaretPosition candidate;
   for (const auto& child :

third_party/blink/renderer/core/layout/ng/ng_physical_fragment.h

@@ -136,7 +136,8 @@ class CORE_EXPORT NGPhysicalFragment
 
   // Returns the offset relative to the parent fragment's content-box.
   NGPhysicalOffset Offset() const {
-    DCHECK(is_placed_);
+    DCHECK(is_placed_) << "this=" << this << " for layout object "
+                       << layout_object_;
     return offset_;
   }