Converts MruWindowTracker to work with common types

ash/common/shell_window_ids.cc

Index: ash/common/shell_window_ids.cc
diff --git a/ash/common/shell_window_ids.cc b/ash/common/shell_window_ids.cc
index 26281f52a3050c147543a12ce120c521231af545..27fdf89b48dd27855be4b0f51f8bf7a56ee33653 100644
--- a/ash/common/shell_window_ids.cc
+++ b/ash/common/shell_window_ids.cc
@@ -27,4 +27,12 @@ const int kActivatableShellWindowIds[] = {
 const size_t kNumActivatableShellWindowIds =
     arraysize(kActivatableShellWindowIds);
 
+bool IsActivatableShellWindowId(int id) {
+  for (size_t i = 0; i < kNumActivatableShellWindowIds; i++) {
+    if (id == kActivatableShellWindowIds[i])
+      return true;
+  }
+  return false;
+}
+
 }  // namespace ash