Fix a ref on a temporary in benchmark_wangs_formula_cubic_log2

Change-Id: Ib68e8f918b7083d0b7934ffeb580fbf9b2a19032
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/362478
Auto-Submit: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>

bench/TessellateBench.cpp

@@ -82,7 +82,7 @@ protected:
     SkString fName;
     std::unique_ptr<GrMockOpTarget> fTarget;
     const SkPath fPath;
-    const SkMatrix& fMatrix;
+    const SkMatrix fMatrix;
 };
 
 #define DEF_PATH_TESS_BENCH(NAME, PATH, MATRIX) \