[LayoutNG] Remove unused variable in layout_block_flow_line.cc

This is a follow-up from:
https://chromium-review.googlesource.com/c/chromium/src/+/583977
which removed unused InlineBox::HasSelectedChildren().
The patch left an unused variable |root_has_selected_children|
dangling.
This patch removed that.

Bug: 789870
Change-Id: I6f5d4cf4f50630e558a314782212949ccd5ad2bc
Reviewed-on: https://chromium-review.googlesource.com/1159555Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580089}

third_party/blink/renderer/core/layout/layout_block_flow_line.cc

@@ -286,7 +286,6 @@ RootInlineBox* LayoutBlockFlow::ConstructLine(BidiRunList<BidiRun>& bidi_runs,
                                               const LineInfo& line_info) {
   DCHECK(bidi_runs.FirstRun());
 
-  bool root_has_selected_children = false;
   InlineFlowBox* parent_box = nullptr;
   int run_count = bidi_runs.RunCount() - line_info.RunsFromLeadingWhitespace();
   for (BidiRun* r = bidi_runs.FirstRun(); r; r = r->Next()) {
@@ -312,10 +311,6 @@ RootInlineBox* LayoutBlockFlow::ConstructLine(BidiRunList<BidiRun>& bidi_runs,
     if (!box)
       continue;
 
-    if (!root_has_selected_children &&
-        box->GetLineLayoutItem().GetSelectionState() != SelectionState::kNone)
-      root_has_selected_children = true;
-
     // If we have no parent box yet, or if the run is not simply a sibling,
     // then we need to construct inline boxes as necessary to properly enclose
     // the run's inline box. Segments can only be siblings at the root level, as