gfx: Add GpuFence::Wait() method

Provide a GpuFence::Wait() method to hide the implementation details of
waiting for a GpuFence object.

Bug: 828393
Change-Id: I2153ebcb4b8cca6bb43c827fb0a9740f6a6cc221
Reviewed-on: https://chromium-review.googlesource.com/1021513
Reviewed-by: David Reveman <reveman@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Daniele Castagna <dcastagna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559292}
ClientGpuFence GpuFence::AsClientGpuFence() {
  return reinterpret_cast<ClientGpuFence>(this);
}

ClientGpuFence GpuFence::AsClientGpuFence() {
  return reinterpret_cast<ClientGpuFence>(this);
}
