Avoid heap allocation in SkPictureStateTree constructor

src/core/SkPictureStateTree.h

Index: src/core/SkPictureStateTree.h
===================================================================
--- src/core/SkPictureStateTree.h	(revision 12590)
+++ src/core/SkPictureStateTree.h	(working copy)
@@ -115,7 +115,6 @@
     void appendNode(uint32_t offset);
 
     SkChunkAlloc fAlloc;
-    Node* fRoot;
     // Needed by saveCollapsed() because nodes do not currently store
     // references to their children.  If they did, we could just retrieve the
     // last added child.
@@ -140,6 +139,9 @@
         };
     };
 
+    Node fRoot;
+    SkMatrix fRootMatrix;
+
     typedef SkRefCnt INHERITED;
 };