move annotations to canvas virtual

src/core/SkAnnotationKeys.h

+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkAnnotationKeys_DEFINED
+#define SkAnnotationKeys_DEFINED
+
+#include "SkTypes.h"
+
+class SkAnnotationKeys {
+public:
+    /**
+     *  Returns the canonical key whose payload is a URL
+     */
+    static const char* URL_Key();
+
+    /**
+     *  Returns the canonical key whose payload is the name of a destination to
+     *  be defined.
+     */
+    static const char* Define_Named_Dest_Key();
+
+    /**
+     *  Returns the canonical key whose payload is the name of a destination to
+     *  be linked to.
+     */
+    static const char* Link_Named_Dest_Key();
+};
+
+#endif