Today, scrollbars are managed entirely within the ScrollableArea subclasses FrameView and PaintLayerScrollableArea. This tightly couples them and increases code complexity. It also makes it difficult to change scrollbar behavior reliably or reuse their functionality.
We should move all the creation, management, and destruction out into a separate class. This was started in https://crrev.com/7a119e4a1eef6d3a91e13b8b86cc12f6ca9b8804 but left much code still in FrameView and PLSA. The remaining bits will require smoothing out any differences between how FrameView and PLSA manage scrollbars and moving them into ScrollbarManager.
I've attached a rough sketch of what I think this should look like
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IMG_20161103_142035 (2).jpg
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