Make DocumentLoader::timing return a reference

Source/core/timing/Performance.cpp

Index: Source/core/timing/Performance.cpp
diff --git a/Source/core/timing/Performance.cpp b/Source/core/timing/Performance.cpp
index a8000fa8f17492f286b29f2f605e5a222c5f5e0e..31f457713f7dfbc423648df29d85525580f30654 100644
--- a/Source/core/timing/Performance.cpp
+++ b/Source/core/timing/Performance.cpp
@@ -49,7 +49,7 @@ static const size_t defaultResourceTimingBufferSize = 150;
 Performance::Performance(LocalFrame* frame)
     : DOMWindowProperty(frame)
     , m_resourceTimingBufferSize(defaultResourceTimingBufferSize)
-    , m_referenceTime(frame && frame->host() ? frame->document()->loader()->timing()->referenceMonotonicTime() : 0.0)
+    , m_referenceTime(frame && frame->host() ? frame->document()->loader()->timing().referenceMonotonicTime() : 0.0)
     , m_userTiming(nullptr)
 {
 }