Oilpan: improve ScrollableArea handling.

Source/core/frame/PinchViewport.cpp

Index: Source/core/frame/PinchViewport.cpp
diff --git a/Source/core/frame/PinchViewport.cpp b/Source/core/frame/PinchViewport.cpp
index 4bf57b5f645b9af7b020a791c6e63a6f6956ec30..1c1fc2e51ca7c89b5e717cf2f347645d4679169b 100644
--- a/Source/core/frame/PinchViewport.cpp
+++ b/Source/core/frame/PinchViewport.cpp
@@ -68,7 +68,7 @@ using blink::GraphicsLayerFactory;
 namespace blink {
 
 PinchViewport::PinchViewport(FrameHost& owner)
-    : m_frameHost(owner)
+    : m_frameHost(&owner)
     , m_scale(1)
     , m_topControlsAdjustment(0)
 {
@@ -79,6 +79,12 @@ PinchViewport::~PinchViewport()
 {
 }
 
+DEFINE_TRACE(PinchViewport)
+{
+    visitor->trace(m_frameHost);
+    ScrollableArea::trace(visitor);
+}
+
 void PinchViewport::setSize(const IntSize& size)
 {
     // When the main frame is remote, we won't have an associated frame.