Implement SkGLContext swapBuffers with fence syncs

src/gpu/SkGpuFenceSync.h

Index: src/gpu/SkGpuFenceSync.h
diff --git a/src/gpu/SkGpuFenceSync.h b/src/gpu/SkGpuFenceSync.h
new file mode 100644
index 0000000000000000000000000000000000000000..b78398fed85736a1236cbc0a11ad54e05c894ed4
--- /dev/null
+++ b/src/gpu/SkGpuFenceSync.h
@@ -0,0 +1,29 @@
+
+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+#ifndef SkGpuFenceSync_DEFINED
+#define SkGpuFenceSync_DEFINED
+
+#include "SkTypes.h"
+
+typedef void* SkPlatformGpuFence;
+
+/*
+ * This class provides an interface to interact with fence syncs. A fence sync is an object that the
+ * client can insert into the GPU command stream, and then at any future time, wait until all
+ * commands that were issued before the fence have completed.
+ */
+class SkGpuFenceSync {
+public:
+    virtual SkPlatformGpuFence SK_WARN_UNUSED_RESULT insertFence() const = 0;
+    virtual bool flushAndWaitFence(SkPlatformGpuFence) const = 0;
+    virtual void deleteFence(SkPlatformGpuFence) const = 0;
+
+    virtual ~SkGpuFenceSync() {}
+};
+
+#endif