Move SkSpan to include/, for use in public API

Change-Id: I674f038600afd6d49316c1ece515941ee5579068
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/406939
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
diff --git a/src/utils/SkShaperJSONWriter.h b/src/utils/SkShaperJSONWriter.h
index bca0cd1..2b94094 100644
--- a/src/utils/SkShaperJSONWriter.h
+++ b/src/utils/SkShaperJSONWriter.h
@@ -14,8 +14,8 @@
 #include <string>
 #include <vector>
 
+#include "include/core/SkSpan.h"
 #include "modules/skshaper/include/SkShaper.h"
-#include "src/core/SkSpan.h"
 
 class SkJSONWriter;