firefox 37.0.2 nsLayoutUtils

static nsSize
ComputeConcreteObjectSize(const nsSize& aConstraintSize,
                          const IntrinsicSize& aIntrinsicSize,
                          const nsSize& aIntrinsicRatio,
                          uint8_t aObjectFit)
{
  // Handle default behavior (filling the container) w/ fast early return.
  // (Also: if there's no valid intrinsic ratio, then we have the "fill"
  // behavior & just use the constraint size.)
  if (MOZ_LIKELY(aObjectFit == NS_STYLE_OBJECT_FIT_FILL) ||
      aIntrinsicRatio.width == 0 ||
      aIntrinsicRatio.height == 0) {
    return aConstraintSize;
  }

  Maybe<nsSize> noneSize;
  Maybe<nsSize> constrainedSize;
  Maybe<nsImageRenderer::FitType> fitType;

  switch (aObjectFit) {
    case NS_STYLE_OBJECT_FIT_NONE:
      noneSize.emplace(ComputeObjectFitNoneSize(aConstraintSize, aIntrinsicSize,
                                         aIntrinsicRatio));
      if (noneSize)
        return *noneSize;
      fitType.emplace(nsImageRenderer::CONTAIN);

    case NS_STYLE_OBJECT_FIT_SCALE_DOWN:
      fitType.emplace(nsImageRenderer::CONTAIN);
      constrainedSize->width =
        std::min(constrainedSize->width, noneSize->width);
      constrainedSize->height =
        std::min(constrainedSize->height, noneSize->height);

    case NS_STYLE_OBJECT_FIT_COVER:
      fitType.emplace(nsImageRenderer::COVER);

    case NS_STYLE_OBJECT_FIT_CONTAIN:
      fitType.emplace(nsImageRenderer::CONTAIN);
  }
  constrainedSize.emplace(nsImageRenderer::ComputeConstrainedSize(aConstraintSize,
                                         aIntrinsicRatio, *fitType));

  if (constrainedSize) {
    return *constrainedSize;
  } else {
    MOZ_ASSERT_UNREACHABLE("Unexpected enum value for 'object-fit'");
    return aConstraintSize; // fall back to (default) 'fill' behavior
  }

}