Use DEFINE_TRACE in FrameSelection instead of DEFINE_VIRTUAL_TRACE

This patch changes |FrameSelection| class to use |DEFINE_TRACE()| instead of
|DEFINE_VIRTUAL_TRACE()| since |FrameSelection| is final.

|DEFINE_VIRTUAL_TRACE()| was introduced by [1] as result of mechanical replacing
of |virtual void trace()|.

[1] https://chromium.googlesource.com/chromium/src/+/0ae5a4a2770e78d7cdc0025f1ebeebd797691b16

BUG=n/a
TEST=n/a; no behavior changes.

Review-Url: https://codereview.chromium.org/2443883002
Cr-Commit-Position: refs/heads/master@{#427030}

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

@@ -290,7 +290,7 @@ class CORE_EXPORT FrameSelection final
   // to use updated selection, we should make |updateIfNeeded()| private.
   void updateIfNeeded();
 
-  DECLARE_VIRTUAL_TRACE();
+  DECLARE_TRACE();
 
  private:
   friend class FrameSelectionTest;