prepare |SVGDocumentExtensions::m_relativeLengthSVGRoots| for oilpan

Source/core/svg/SVGDocumentExtensions.h

Index: Source/core/svg/SVGDocumentExtensions.h
diff --git a/Source/core/svg/SVGDocumentExtensions.h b/Source/core/svg/SVGDocumentExtensions.h
index 1ceb7ab3a494f38a1df08d364f82a5d0e12ebb9a..d43a8fdb8baaf5b4ac0727faa7873c6e09f93929 100644
--- a/Source/core/svg/SVGDocumentExtensions.h
+++ b/Source/core/svg/SVGDocumentExtensions.h
@@ -110,7 +110,7 @@ private:
     HashMap<AtomicString, OwnPtr<SVGPendingElements> > m_pendingResourcesForRemoval; // Resources that are pending and scheduled for removal.
     HashMap<SVGElement*, OwnPtr<HashSet<SVGElement*> > > m_elementDependencies;
     OwnPtr<SVGResourcesCache> m_resourcesCache;
-    HashSet<SVGSVGElement*> m_relativeLengthSVGRoots; // Root SVG elements with relative length descendants.
+    WillBeHeapHashSet<RawPtrWillBeWeakMember<SVGSVGElement> > m_relativeLengthSVGRoots; // Root SVG elements with relative length descendants.

[haraken, 2014/06/09 09:37:55]

Just help me understand: How is the weak processing implemented in non-oilpan
builds? I couldn't find code that calls
removeSVGRootWithRelativeLengthDescendents from some SVGSVGElement's destructor.

(It makes sense to make this a hash set of weak members, but I'd like to
understand the background.)

[kouhei (in TOK), 2014/06/09 09:53:46]

if (document is dead && <svg> is dead) the stale ptr in hashset is deleted
anyway
if (document is alive && <svg> is dead) <svg>::removedFrom handles this
if (document is dead && <svg> is alive) the stale ptr in hashset is deleted
anyway 
if (document is alive && <svg> is alive) no need to remove

[kouhei (in TOK), 2014/06/10 05:24:37]

Discussed offline. Made this a strong ref to be explicit that this map never
outlive its referenced elements.

     FloatPoint m_translate;
 #if !ASSERT_DISABLED
     bool m_inRelativeLengthSVGRootsInvalidation;