Avoid redundant equality check in SelectionEditor::SetSelection

SelectionEditor::SetSelection is only called from FrameSelection
SetSelection, which does the equality of the old DOM selection with
the existing selection. So we can avoid doing the check in Selection
Editor::SetSelection

BUG=N/A

Change-Id: Ida77fee4e4bd26aa5729d367a181b7437c7eaaf9
Reviewed-on: https://chromium-review.googlesource.com/595787
Commit-Queue: Tanvir Rizvi <tanvir.rizvi@samsung.com>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491296}

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

@@ -113,8 +113,7 @@ void SelectionEditor::MarkCacheDirty() {
 
 void SelectionEditor::SetSelection(const SelectionInDOMTree& new_selection) {
   new_selection.AssertValidFor(GetDocument());
-  if (selection_ == new_selection)
-    return;
+  DCHECK_NE(selection_, new_selection);
   ClearDocumentCachedRange();
   MarkCacheDirty();
   selection_ = new_selection;