Add support for flattening in GeometryMapper.

third_party/WebKit/Source/platform/graphics/paint/GeometryMapperTransformCache.h

Index: third_party/WebKit/Source/platform/graphics/paint/GeometryMapperTransformCache.h
diff --git a/third_party/WebKit/Source/platform/graphics/paint/GeometryMapperTransformCache.h b/third_party/WebKit/Source/platform/graphics/paint/GeometryMapperTransformCache.h
index 9d08f724bed344abfeb86ceb6547039eb03e26c3..7b82beca612cdfef75d62ce2fbd099b216d24050 100644
--- a/third_party/WebKit/Source/platform/graphics/paint/GeometryMapperTransformCache.h
+++ b/third_party/WebKit/Source/platform/graphics/paint/GeometryMapperTransformCache.h
@@ -24,6 +24,8 @@ class PLATFORM_EXPORT GeometryMapperTransformCache {
   // Returns the transformed rect (see GeometryMapper.h) of |this| in the
   // space of |ancestorTransform|, if there is one cached. Otherwise returns
   // null.
+  //
+  // These transforms are not flattened to 2d.
   const TransformationMatrix* getCachedTransform(
       const TransformPaintPropertyNode* ancestorTransform);