[LayoutNG] Mark LayoutSelection::LayoutSelectionStartEndForNG const.

This patch marks LayoutSelection::LayoutSelectionStartEndForNG
and FrameSelection::LayoutSelectionStartEndForNG const because of
the function constness.

Bug: 708452
Change-Id: I462fa178a9c656f184f43aac62eedff552290219
Reviewed-on: https://chromium-review.googlesource.com/892943Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Yoichi Osato <yoichio@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533171}

third_party/WebKit/Source/core/editing/FrameSelection.cpp

@@ -1255,7 +1255,7 @@ void FrameSelection::ClearLayoutSelection() {
 }
 
 std::pair<unsigned, unsigned> FrameSelection::LayoutSelectionStartEndForNG(
-    const NGPhysicalTextFragment& text_fragment) {
+    const NGPhysicalTextFragment& text_fragment) const {
   return layout_selection_->SelectionStartEndForNG(text_fragment);
 }
 

third_party/WebKit/Source/core/editing/FrameSelection.h

@@ -225,7 +225,7 @@ class CORE_EXPORT FrameSelection final
   WTF::Optional<unsigned> LayoutSelectionEnd() const;
   void ClearLayoutSelection();
   std::pair<unsigned, unsigned> LayoutSelectionStartEndForNG(
-      const NGPhysicalTextFragment&);
+      const NGPhysicalTextFragment&) const;
 
   void Trace(blink::Visitor*);
 

third_party/WebKit/Source/core/editing/LayoutSelection.cpp

@@ -662,7 +662,7 @@ static unsigned ClampOffset(unsigned offset,
 }
 
 std::pair<unsigned, unsigned> LayoutSelection::SelectionStartEndForNG(
-    const NGPhysicalTextFragment& text_fragment) {
+    const NGPhysicalTextFragment& text_fragment) const {
   // FrameSelection holds selection offsets in layout block flow at
   // LayoutSelection::Commit() if selection starts/ends within Text that
   // each LayoutObject::SelectionState indicates.

third_party/WebKit/Source/core/editing/LayoutSelection.h

@@ -110,7 +110,7 @@ class LayoutSelection final : public GarbageCollected<LayoutSelection> {
   // (text_fragment.StartOffset(), text_fragment.EndOffset()).
   // If first equals second, it indicates "no selection in fragment".
   std::pair<unsigned, unsigned> SelectionStartEndForNG(
-      const NGPhysicalTextFragment&);
+      const NGPhysicalTextFragment&) const;
 
   void OnDocumentShutdown();