There are currently several frame-counters:

 . CompositorFrameMetadata::frame_token
 . LayerTreeHost::source_frame_number_
 . frame_index in CompositorFrameSinkSupport

They are slightly different from each other, e.g. frame_token starts at 1, and a zero frame-token is invalid. source_frame_number_ starts at 0 (but is sometimes also negative, at least in tests). frame_index starts at 2 (not clear why).

These could all be unified.