[LayoutNG] Minor clean up in PositionWithParentBfc() & co.

Rename the method to PositionEmptyChildWithParentBfc(), and make it a
const method. Also get rid of unused and unimplemented
PositionWithBfcOffset(const NGBfcOffset&).

Also some fixes in comments and parameter names, and added some comments
on my own.

Moved a DCHECK(is_empty_block) to also hit when
PositionEmptyChildWithParentBfc(), just to be clear about the fact that
the block has to be empty at this point.

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: I7d171aa40d3f3a5705f7a0126725a7905d37c504
Reviewed-on: https://chromium-review.googlesource.com/955645Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542197}

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

@@ -973,12 +973,18 @@ bool NGBlockLayoutAlgorithm::HandleInflow(
     if (!PositionWithBfcOffset(layout_result->BfcOffset().value(),
                                &child_bfc_offset))
       return false;
-  } else if (container_builder_.BfcOffset()) {
-    child_bfc_offset =
-        PositionWithParentBfc(child, *child_space, child_data, *layout_result,
-                              &empty_block_affected_by_clearance);
-  } else
+  } else {
+    // Layout wasn't able to determine the BFC offset of the child. This has to
+    // mean that the child is empty (block-size-wise).
     DCHECK(is_empty_block);
+    if (container_builder_.BfcOffset()) {
+      // Since we know our own BFC offset, though, we can calculate that of the
+      // child as well.
+      child_bfc_offset = PositionEmptyChildWithParentBfc(
+          child, *child_space, child_data, *layout_result,
+          &empty_block_affected_by_clearance);
+    }
+  }
 
   // We need to re-layout a child if it was affected by clearance in order to
   // produce a new margin strut. For example:
@@ -1194,12 +1200,12 @@ bool NGBlockLayoutAlgorithm::PositionWithBfcOffset(
   return true;
 }
 
-NGBfcOffset NGBlockLayoutAlgorithm::PositionWithParentBfc(
+NGBfcOffset NGBlockLayoutAlgorithm::PositionEmptyChildWithParentBfc(
     const NGLayoutInputNode& child,
-    const NGConstraintSpace& space,
+    const NGConstraintSpace& child_space,
     const NGInflowChildData& child_data,
     const NGLayoutResult& layout_result,
-    bool* empty_block_affected_by_clearance) {
+    bool* has_clearance) const {
   DCHECK(IsEmptyBlock(child, layout_result));
 
   // The child must be an in-flow zero-block-size fragment, use its end margin
@@ -1217,8 +1223,8 @@ NGBfcOffset NGBlockLayoutAlgorithm::PositionWithParentBfc(
                                child_available_size_.inline_size);
   }
 
-  *empty_block_affected_by_clearance =
-      AdjustToClearance(space.ClearanceOffset(), &child_bfc_offset);
+  *has_clearance =
+      AdjustToClearance(child_space.ClearanceOffset(), &child_bfc_offset);
 
   return child_bfc_offset;
 }

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

@@ -82,23 +82,22 @@ class CORE_EXPORT NGBlockLayoutAlgorithm
       bool empty_block_affected_by_clearance);
 
   // Positions the fragment that knows its BFC offset.
-  WTF::Optional<NGBfcOffset> PositionWithBfcOffset(
-      const NGBfcOffset& bfc_offset);
   bool PositionWithBfcOffset(const NGBfcOffset& bfc_offset,
                              WTF::Optional<NGBfcOffset>* child_bfc_offset);
 
-  // Positions using the parent BFC offset.
-  // Fragment doesn't know its offset but we can still calculate its BFC
+  // Position an empty child using the parent BFC offset.
+  // The fragment doesn't know its offset, but we can still calculate its BFC
   // position because the parent fragment's BFC is known.
   // Example:
   //   BFC Offset is known here because of the padding.
   //   <div style="padding: 1px">
-  //     <div id="empty-div" style="margins: 1px"></div>
-  NGBfcOffset PositionWithParentBfc(const NGLayoutInputNode& child,
-                                    const NGConstraintSpace&,
-                                    const NGInflowChildData& child_data,
-                                    const NGLayoutResult&,
-                                    bool* empty_block_affected_by_clearance);
+  //     <div id="empty-div" style="margin: 1px"></div>
+  NGBfcOffset PositionEmptyChildWithParentBfc(
+      const NGLayoutInputNode& child,
+      const NGConstraintSpace& child_space,
+      const NGInflowChildData& child_data,
+      const NGLayoutResult&,
+      bool* has_clearance) const;
 
   void HandleOutOfFlowPositioned(const NGPreviousInflowPosition&, NGBlockNode);
   void HandleFloat(const NGPreviousInflowPosition&,