Allow gpu ResourceHandle class to be shared for multiple purposes

src/gpu/GrResourceHandle.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 GrResourceHandle_DEFINED
+#define GrResourcehandle_DEFINED
+
+#include "SkTypes.h"
+
+// Opaque handle to a resource. Users should always use the macro below to create a specific indexed
+// version of the class. No function should ever take a generic GrResourceHandle and instead use

[bsalomon, 2016/05/06 18:40:56]

Maybe remove the last sentence? One of the benefits of the template approach is
that there no longer is a base class.

+// a specific type indexed one.
+template <typename kind> class GrResourceHandle {
+public:
+    GrResourceHandle(int value) : fValue(value) {
+        SkASSERT(this->isValid());
+    }
+
+    GrResourceHandle() : fValue(kInvalid_ResourceHandle) {}
+
+    bool operator==(const GrResourceHandle& other) const { return other.fValue == fValue; }
+    bool isValid() const { return kInvalid_ResourceHandle != fValue; }
+    int toIndex() const { SkASSERT(this->isValid()); return fValue; }
+
+private:
+    static const int kInvalid_ResourceHandle = -1;
+    int fValue;
+};
+
+// Creates a type "name" that is a specfic type index of GrResourceHandle.
+#define GR_DEFINE_RESOURCE_HANDLE_CLASS(name) \
+    struct name##Kind {};  \
+    using name = GrResourceHandle<name##Kind>;
+#endif