Allow subsequence caching for SVG roots that have PaintLayers.

third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp

Index: third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp b/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
index eea4ca3e7b43624d3f5d5a8380a852969219642a..a3b2ef5b3824f9df8e274247ed5e6a00d067b623 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
@@ -151,13 +151,15 @@ static bool shouldCreateSubsequence(const PaintLayer& paintLayer,
     return false;
 
   // Create subsequence for only stacking contexts whose painting are atomic.
-  if (!paintLayer.stackingNode()->isStackingContext())
+  if (!paintLayer.stackingNode()->isStackingContext() &&
+      !paintLayer.layoutObject()->isSVGRoot())

[wkorman, 2016/11/12 00:05:51]

Consider adding comment above this block re: why we allow for SVGRoot here and
below.

[chrishtr, 2016/11/12 00:15:20]

Done.

     return false;
 
   // The layer doesn't have children. Subsequence caching is not worth because
   // normally the actual painting will be cheap.
   if (!PaintLayerStackingNodeIterator(*paintLayer.stackingNode(), AllChildren)
-           .next())
+           .next() &&
+      !paintLayer.layoutObject()->isSVGRoot())
     return false;
 
   // When in FOUC-avoidance mode, don't cache any subsequences, to avoid having