SkRecord: infer return type for visit() and mutate().

Review URL: https://codereview.chromium.org/1824983003
diff --git a/tests/RecorderTest.cpp b/tests/RecorderTest.cpp
index b1af138..59d5a8e 100644
--- a/tests/RecorderTest.cpp
+++ b/tests/RecorderTest.cpp
@@ -31,7 +31,7 @@
 
     void apply(const SkRecord& record) {
         for (int i = 0; i < record.count(); i++) {
-            record.visit<void>(i, *this);
+            record.visit(i, *this);
         }
     }