message: Add missing `override'
  This fixes a warning produced by clang:
  ./tensorflow/core/common_runtime/gpu/gpu_device.h:70:10: warning: 'FillContextMap' overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
  Status FillContextMap(const Graph* graph,
  ^
  ./tensorflow/core/common_runtime/device.h:124:18: note: overridden virtual function is here
  virtual Status FillContextMap(const Graph* graph,
diff: @@ -68,7 +68,7 @@ class BaseGPUDevice : public LocalDevice {
  const TensorReferenceVector& tensor_refs) override;
  Status FillContextMap(const Graph* graph,
  - DeviceContextMap* device_context_map);
  + DeviceContextMap* device_context_map) override;
  void Compute(OpKernel* op_kernel, OpKernelContext* context) override;
  