Add fence support for TransferBuffers

src/gpu/GrGpu.h

Index: src/gpu/GrGpu.h
diff --git a/src/gpu/GrGpu.h b/src/gpu/GrGpu.h
index 3e3a4cd676b0388ef72b41def0fc3f7d6f5e0256..b8703dc68e364fd15fe6e025ba75aad39dc37df2 100644
--- a/src/gpu/GrGpu.h
+++ b/src/gpu/GrGpu.h
@@ -305,7 +305,7 @@ public:
     bool transferPixels(GrSurface* surface,
                         int left, int top, int width, int height,
                         GrPixelConfig config, GrBuffer* transferBuffer,
-                        size_t offset, size_t rowBytes);
+                        size_t offset, size_t rowBytes, GrFence* fence);
 
     /**
      * This is can be called before allocating a texture to be a dst for copySurface. This is only
@@ -371,6 +371,10 @@ public:
     // Provides a hook for post-flush actions (e.g. PLS reset and Vulkan command buffer submits).
     virtual void finishDrawTarget() {}
 
+    virtual GrFence SK_WARN_UNUSED_RESULT insertFence() const = 0;
+    virtual bool waitFence(GrFence, uint64_t timeout = 1000) const = 0;
+    virtual void deleteFence(GrFence) const = 0;
+
     ///////////////////////////////////////////////////////////////////////////
     // Debugging and Stats