Remove unimplemented method from FrameSelection.h

FrameSelection.h contains a declaration for an UpdateSelectionIfNeeded() method,
whose implementation has since been removed. This CL removes the unused
declaration.

Change-Id: Ic0410d804e7df32e7ed7d3df8371e1cb2a2672ed
Reviewed-on: https://chromium-review.googlesource.com/847698Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Ryan Landay <rlanday@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526576}

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

@@ -254,11 +254,6 @@ class CORE_EXPORT FrameSelection final
 
   void SetUseSecureKeyboardEntry(bool);
 
-  void UpdateSelectionIfNeeded(const Position& base,
-                               const Position& extent,
-                               const Position& start,
-                               const Position& end);
-
   GranularityStrategy* GetGranularityStrategy();
 
   IntRect ComputeRectToScroll(RevealExtentOption);