message: Handle staging buffer map failure on Metal.
diff: @@ -25,7 +25,16 @@ namespace dawn_native { namespace metal {
  const size_t bufferSize = GetSize();
  mBuffer = [mDevice->GetMTLDevice() newBufferWithLength:bufferSize
  options:MTLResourceStorageModeShared];
  +
  + if (mBuffer == nil) {
  + return DAWN_CONTEXT_LOST_ERROR("Unable to allocate buffer.");
  + }
  +
  mMappedPointer = [mBuffer contents];
  + if (mMappedPointer == nullptr) {
  + return DAWN_CONTEXT_LOST_ERROR("Unable to map staging buffer.");
  + }
  +
  return {};
  }
  