Selection API: Add a UseCounter for focus change by Selection.modify().

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 2ad77dab5d18b0d974ef531e3cb882879af0194c..5b9f4478aeffe04440a068ad91d83ed2e7a5bbb3 100644
--- a/third_party/WebKit/Source/core/editing/DOMSelection.cpp
+++ b/third_party/WebKit/Source/core/editing/DOMSelection.cpp
@@ -436,7 +436,11 @@ void DOMSelection::modify(const String& alterString,
   else
     return;
 
+  Element* focusedElement = frame()->document()->focusedElement();
   frame()->selection().modify(alter, direction, granularity);
+  if (frame() && frame()->document() &&
+      focusedElement != frame()->document()->focusedElement())
+    UseCounter::count(frame(), UseCounter::SelectionFuncionsChangeFocus);
 }
 
 // https://www.w3.org/TR/selection-api/#dom-selection-extend