[SVG] Reinstate RenderSVGResourcePattern's inval-during-paint guard.

Source/core/rendering/svg/RenderSVGResourcePattern.cpp

Index: Source/core/rendering/svg/RenderSVGResourcePattern.cpp
diff --git a/Source/core/rendering/svg/RenderSVGResourcePattern.cpp b/Source/core/rendering/svg/RenderSVGResourcePattern.cpp
index fa31c0cae1e0a3c2acdbbaa528a5e97b9ed914c5..6bae6c3f2644eccab23783c27c8fac772cb40d8c 100644
--- a/Source/core/rendering/svg/RenderSVGResourcePattern.cpp
+++ b/Source/core/rendering/svg/RenderSVGResourcePattern.cpp
@@ -62,14 +62,15 @@ void RenderSVGResourcePattern::removeClientFromCache(RenderObject* client, bool
 
 PatternData* RenderSVGResourcePattern::patternForRenderer(const RenderObject& object)
 {
-    auto addResult = m_patternMap.add(&object, nullptr);
-    OwnPtr<PatternData>& patternData = addResult.storedValue->value;
+    ASSERT(!m_shouldCollectPatternAttributes);
 
-    if (addResult.isNewEntry)
-        patternData = buildPatternData(object);
+    // FIXME: the double hash lookup is needed to guard against paint-time invalidation
+    // (painting animated images may trigger layout invals which delete our map entry).
+    // Hopefully that will be addressed at some point, and then we can optimize the lookup.
+    if (PatternData* currentData = m_patternMap.get(&object))
+        return currentData;
 
-    ASSERT(!m_shouldCollectPatternAttributes);
-    return patternData.get();
+    return m_patternMap.set(&object, buildPatternData(object)).storedValue->value.get();
 }
 
 PassOwnPtr<PatternData> RenderSVGResourcePattern::buildPatternData(const RenderObject& object)