[LayoutNG] Avoid creating empty LineBoxData in NGLineBuilder

Empty LineBoxData produces empty BidiRunList, which confuses
LayoutBlockFlow::constructLine() in CopyFragmentDataToLayoutBlockFlow().

There are cases where we need empty line boxes, such as when the caret
is set while editing. Such cases need to be revisited in future.

This is part of the effort to layout Wikipedia main page[1] with
LayoutNGInline runtime flag enabled.

[1] https://en.wikipedia.org/wiki/Main_Page

BUG=636993

Review-Url: https://codereview.chromium.org/2676883002
Cr-Commit-Position: refs/heads/master@{#448099}

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

@@ -77,14 +77,16 @@ void NGLineBuilder::CreateLine() {
   }
   DCHECK_EQ(fragments_.size(), offsets_.size());
 
-  line_box_data_list_.grow(line_box_data_list_.size() + 1);
-  LineBoxData& line_box_data = line_box_data_list_.back();
-  line_box_data.fragment_end = fragments_.size();
-  line_box_data.inline_size = inline_offset;
-
-  max_inline_size_ = std::max(max_inline_size_, inline_offset);
-  // TODO(kojii): Implement block size when we support baseline alignment.
-  content_size_ += LayoutUnit(100);
+  if (!fragments_.isEmpty()) {
+    line_box_data_list_.grow(line_box_data_list_.size() + 1);
+    LineBoxData& line_box_data = line_box_data_list_.back();
+    line_box_data.fragment_end = fragments_.size();
+    line_box_data.inline_size = inline_offset;
+
+    max_inline_size_ = std::max(max_inline_size_, inline_offset);
+    // TODO(kojii): Implement block size when we support baseline alignment.
+    content_size_ += LayoutUnit(100);
+  }
 
   line_item_chunks_.clear();
 #if DCHECK_IS_ON()