Add support for flattening in GeometryMapper.

third_party/WebKit/Source/platform/graphics/paint/GeometryMapper.cpp

Index: third_party/WebKit/Source/platform/graphics/paint/GeometryMapper.cpp
diff --git a/third_party/WebKit/Source/platform/graphics/paint/GeometryMapper.cpp b/third_party/WebKit/Source/platform/graphics/paint/GeometryMapper.cpp
index 6258d642b44163697b80df40d37b24c93ed689c8..55f018cd203b269f739b2cfb1773f64fbb530436 100644
--- a/third_party/WebKit/Source/platform/graphics/paint/GeometryMapper.cpp
+++ b/third_party/WebKit/Source/platform/graphics/paint/GeometryMapper.cpp
@@ -373,6 +373,14 @@ const TransformationMatrix& GeometryMapper::localToAncestorMatrixInternal(
        it++) {
     TransformationMatrix localTransformMatrix = (*it)->matrix();
     localTransformMatrix.applyTransformOrigin((*it)->origin());
+
+    // Flattening Lemma: flatten(A * flatten(B)) = flatten(flatten(A) * B).
+    // goo.gl/DNKyOc. Thus we can flatten transformMatrix rather than
+    // localTransformMatrix, because GeometryMapper only supports transforms
+    // into a flattened destination space.
+    if ((*it)->flattensInheritedTransform())
+      transformMatrix.flattenTo2d();
+
     transformMatrix = transformMatrix * localTransformMatrix;
     (*it)->getTransformCache().setCachedTransform(ancestorTransformNode,
                                                   transformMatrix);