Add defensive code for now to RenderLayerRepainter to prevent squashing crashes

Source/core/rendering/RenderLayerRepainter.cpp

Index: Source/core/rendering/RenderLayerRepainter.cpp
diff --git a/Source/core/rendering/RenderLayerRepainter.cpp b/Source/core/rendering/RenderLayerRepainter.cpp
index ad8c16b306c2926dc3ef3ecb53adda788dc144cc..b29c395781968d0dbb637dd7b49b1b14df192ea7 100644
--- a/Source/core/rendering/RenderLayerRepainter.cpp
+++ b/Source/core/rendering/RenderLayerRepainter.cpp
@@ -203,7 +203,18 @@ void RenderLayerRepainter::setBackingNeedsRepaintInRect(const LayoutRect& r)
     if (m_renderer.compositingState() == PaintsIntoGroupedBacking) {
         LayoutRect updatedRect(r);
 
-        RenderLayerModelObject* transformedAncestor = m_renderer.layer()->enclosingTransformedAncestor()->renderer();
+        ASSERT(m_renderer.layer());
+        ASSERT(m_renderer.layer()->enclosingTransformedAncestor());
+        ASSERT(m_renderer.layer()->enclosingTransformedAncestor()->renderer());
+
+        // FIXME: this defensive code should not have to exist. None of these pointers should ever be 0. See crbug.com/370410.
+        RenderLayerModelObject* transformedAncestor = 0;
+        if (RenderLayer* layer = m_renderer.layer()) {

[abarth-chromium, 2014/05/08 16:59:34]

I don't think it's worth null checking m_renderer.layer().  We dereference that
constantly without a null check.

[chrishtr, 2014/05/08 17:02:24]

Done.

+            if (RenderLayer* ancestor = layer->enclosingTransformedAncestor())
+                transformedAncestor = ancestor->renderer();
+        }
+        if (!transformedAncestor)
+            return;
 
         // If the transformedAncestor is actually the RenderView, we might get
         // confused and think that we can use LayoutState. Ideally, we'd made