Remove usage of ExecutionContextClient frame-variants from DOMSelection

Change-Id: I7d0598be2359323a33915fff8c4345eeac8a2a3b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2490546
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Auto-Submit: Nate Chapin <japhet@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819636}

third_party/blink/renderer/core/editing/dom_selection.h

@@ -40,6 +40,7 @@
 namespace blink {
 
 class ExceptionState;
+class FrameSelection;
 class Node;
 class Range;
 class SetSelectionOptions;
@@ -102,6 +103,7 @@ class CORE_EXPORT DOMSelection final : public ScriptWrappable,
   void Trace(Visitor*) const override;
 
  private:
+  FrameSelection& Selection() const;
   bool IsAvailable() const;
 
   void UpdateFrameSelection(const SelectionInDOMTree&,