The API has changed for the slice config of SSpatialLayerConfig as of
OpenH264 v1.6. Update H264EncoderImpl with an ifdef that uses the
correct API depending on what version of OpenH264 is being used.
BUG=webrtc:6583
Review-Url: https://codereview.webrtc.org/2440113002
Cr-Commit-Position: refs/heads/master@{#14762}