Selection API: Add a UseCounter for focus change by selection change.

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

Index: third_party/WebKit/Source/core/editing/DOMSelection.cpp
diff --git a/third_party/WebKit/Source/core/editing/DOMSelection.cpp b/third_party/WebKit/Source/core/editing/DOMSelection.cpp
index b19285ebdf988df34cb5022b386dccb7f546b0a4..c4202be576ea902eda0fa0c576ab3d3867a0c0c5 100644
--- a/third_party/WebKit/Source/core/editing/DOMSelection.cpp
+++ b/third_party/WebKit/Source/core/editing/DOMSelection.cpp
@@ -82,8 +82,12 @@ void DOMSelection::updateFrameSelection(const SelectionInDOMTree& selection,
   // TODO(tkent): Specify FrameSelection::DoNotSetFocus. crbug.com/690272
   bool didSet = frameSelection.setSelectionDeprecated(selection);
   cacheRangeIfSelectionOfDocument(newCachedRange);
-  if (didSet)
-    frameSelection.didSetSelectionDeprecated();
+  if (!didSet)
+    return;
+  Element* focusedElement = frame()->document()->focusedElement();
+  frameSelection.didSetSelectionDeprecated();
+  if (focusedElement != frame()->document()->focusedElement())

[tkent, 2017/02/28 07:14:42]

This line crashed because frame() or frame()->document() can be null after focus
events.

+    UseCounter::count(frame(), UseCounter::SelectionFuncionsChangeFocus);
 }
 
 const VisibleSelection& DOMSelection::visibleSelection() const {