Compute margin block start for 1st block in LayoutNG root constraint space

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

Index: third_party/WebKit/Source/core/layout/ng/ng_block_layout_algorithm.cc
diff --git a/third_party/WebKit/Source/core/layout/ng/ng_block_layout_algorithm.cc b/third_party/WebKit/Source/core/layout/ng/ng_block_layout_algorithm.cc
index 2c9789afc08cec3ded1ab824f738533f80f753cb..fcd57428d2fa9cad658bf54f561a969cec5de6d8 100644
--- a/third_party/WebKit/Source/core/layout/ng/ng_block_layout_algorithm.cc
+++ b/third_party/WebKit/Source/core/layout/ng/ng_block_layout_algorithm.cc
@@ -160,15 +160,18 @@ LayoutUnit NGBlockLayoutAlgorithm::CollapseMargins(
     curr_margin_strut.SetMarginBlockEnd(margins.block_end);
   }
 
-  // Update the parent fragment's margin strut
-  UpdateMarginStrut(curr_margin_strut);
-
-  // Compute the margin block start for adjoining blocks.
+  // Compute the margin block start for
+  // 1) adjoining blocks
+  // 2) 1st block in LayoutNG root element's constraint space.
   LayoutUnit margin_block_start;
-  if (is_fragment_margin_strut_block_start_updated_) {
+  if (is_fragment_margin_strut_block_start_updated_ || space.IsRoot()) {
     margin_block_start = ComputeCollapsedMarginBlockStart(
         prev_child_margin_strut_, curr_margin_strut);
   }
+
+  // Update the parent fragment's margin strut
+  UpdateMarginStrut(curr_margin_strut);
+
   prev_child_margin_strut_ = curr_margin_strut;
   return margin_block_start;
 }