blink_gc_plugin: detect singletons with embedded ScriptWrappables.

tools/clang/blink_gc_plugin/CMakeLists.txt

 set(LIBRARYNAME BlinkGCPlugin)
 
 set(plugin_sources
   BlinkGCPlugin.cpp
   BlinkGCPluginConsumer.cpp
   CheckDispatchVisitor.cpp
   CheckFieldsVisitor.cpp
   CheckFinalizerVisitor.cpp
   CheckGCRootsVisitor.cpp
+  CheckSingletonVisitor.cpp
   CheckTraceVisitor.cpp
   CollectVisitor.cpp
   Config.cpp
   DiagnosticsReporter.cpp
   Edge.cpp
   RecordInfo.cpp)
 
 if(WIN32)
   # Clang doesn't support loadable modules on Windows. Unfortunately, building
   # the plugin as a static library and linking clang against it doesn't work.
@@ skipping 30 matching lines @@
     python tests/legacy_naming/test.py
     ${CMAKE_BINARY_DIR}/bin/clang
     $<TARGET_FILE:lib${LIBRARYNAME}>
     )
   cr_add_test(blink_gc_plugin_test
     python tests/test.py
     ${CMAKE_BINARY_DIR}/bin/clang
     $<TARGET_FILE:lib${LIBRARYNAME}>
     )
 endif()