[LayoutNG] Remove extra call to AppendIdentityMapping

This patch removes an extra call to AppendIdentityMapping(),
which will later hit DCHECK in ComputeOffsetMappingIfNeeded().

AppendBreakOpportunity() calls Append(), which calls
AppendIdentityMapping(). The caller of AppendBreakOpportunity()
should not call AppendIdentityMapping().

In addition, this patch adds DCHECK to ensure the integrity check
built into ComputeOffsetMappingIfNeeded() always run.

Bug: 636993
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: Ic7c0ec9d7c8ed8f65475b29bff633ddbeeee3248
Reviewed-on: https://chromium-review.googlesource.com/859817Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#528384}

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

@@ -178,7 +178,6 @@ void NGInlineItemsBuilderTemplate<OffsetMappingBuilder>::
   if (last_collapsible_space_ == CollapsibleSpace::kSpaceNoWrap &&
       IsCollapsibleSpace(string[start]) && style->AutoWrap()) {
     AppendBreakOpportunity(style, layout_object);
-    mapping_builder_.AppendIdentityMapping(1);
     start++;
     last_collapsible_space_ = CollapsibleSpace::kSpace;
   }

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

@@ -497,6 +497,15 @@ void NGInlineNode::PrepareLayoutIfNeeded() {
   DCHECK_EQ(data, MutableData());
 
   block_flow->ClearNeedsCollectInlines();
+
+#if DCHECK_IS_ON()
+  // ComputeOffsetMappingIfNeeded() runs some integrity checks as part of
+  // creating offset mapping. Run the check, and discard the result.
+  DCHECK(!data->offset_mapping_);
+  ComputeOffsetMappingIfNeeded();
+  DCHECK(data->offset_mapping_);
+  data->offset_mapping_.reset();
+#endif
 }
 
 const NGInlineNodeData& NGInlineNode::EnsureData() {