Create stub GrAuditTrail class

src/gpu/GrAuditTrail.cpp

+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "GrAuditTrail.h"
+
+SkString GrAuditTrail::toJson() const {
+    SkString json;
+    json.append("{\n");
+    json.append("Ops: [\n");
+    for (int i = 0; i < fOps.count(); i++) {
+        json.append(fOps[i].toJson());
+        if (i < fOps.count() - 1) {
+            json.append(",\n");
+        }
+    }
+    json.append("]\n");
+    json.append("}\n");
+    return json;
+}
+
+SkString GrAuditTrail::Op::toJson() const {
+    SkString json;
+    json.append("{\n");
+    json.appendf("%s\n", fName.c_str());
+    json.append("}\n");
+    return json;
+}
+