message: Fix leak in Metal's Device::GetPendingCommandBuffer().
  GetPendingCommandBuffer() doesn't need to retain the metal
  command buffer; it's created with a ref.
diff: @@ -152,7 +152,6 @@ namespace backend { namespace metal {
  id<MTLCommandBuffer> Device::GetPendingCommandBuffer() {
  if (mPendingCommands == nil) {
  mPendingCommands = [mCommandQueue commandBuffer];
  - [mPendingCommands retain];
  }
  return mPendingCommands;
  }
  