Oilpan: Ensure that types definitions are known for traced weak members.

R=sigbjornf@opera.com
BUG=

Review URL: https://codereview.chromium.org/334623006

git-svn-id: svn://svn.chromium.org/blink/trunk@176429 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/platform/heap/Visitor.h

@@ -259,6 +259,8 @@ public:
     template<typename T>
     void trace(const WeakMember<T>& t)
     {
+        // Check that we actually know the definition of T when tracing.
+        COMPILE_ASSERT(sizeof(T), WeNeedToKnowTheDefinitionOfTheTypeWeAreTracing);
         registerWeakCell(const_cast<WeakMember<T>&>(t).cell());
     }