Replace raw pointer with UntracedMember

This is a follow-up CL for http://crrev.com/1397073002
static_assert in HashTable ignored this case because
HTMLPlugInElement is forward declared in this file and
we cannot know it is GCed class.

BUG=515524

Review URL: https://codereview.chromium.org/1459473005

Cr-Commit-Position: refs/heads/master@{#360556}

third_party/WebKit/Source/core/frame/LocalFrame.h

@@ -242,8 +242,7 @@ private:
     // weak callback, the set itself is not on the heap and the
     // references are bare pointers (rather than WeakMembers.)
     // See LocalFrame::clearWeakMembers().
-    GC_PLUGIN_IGNORE("553613")
-    HashSet<HTMLPlugInElement*> m_pluginElements;
+    HashSet<UntracedMember<HTMLPlugInElement>> m_pluginElements;
 #endif
 
     float m_pageZoomFactor;