Oilpan: address ScrollableArea-induced build breakage from r194957.

Source/core/frame/FrameHost.cpp

Index: Source/core/frame/FrameHost.cpp
diff --git a/Source/core/frame/FrameHost.cpp b/Source/core/frame/FrameHost.cpp
index 6dccbf6b5e66236b445af2bab12072d7cd033a74..e1fac33872e86df829daf6c209b3efdceeb8bd2d 100644
--- a/Source/core/frame/FrameHost.cpp
+++ b/Source/core/frame/FrameHost.cpp
@@ -104,7 +104,6 @@ DEFINE_TRACE(FrameHost)
 {
     visitor->trace(m_page);
     visitor->trace(m_topControls);
-    visitor->trace(m_pinchViewport);
     visitor->trace(m_eventHandlerRegistry);
     visitor->trace(m_consoleMessageStorage);
 }