In https://chromium-review.googlesource.com/c/chromium/src/+/609317 we started creating scroll hit test layers but these are sized to the overflow clip bounds which do not include scrollbars. The scroll hit test layer should be sized to include scrollbars.

This is a subset of the sizing needed to properly support SPV1 scrolling behavior. We will need to also include borders to match non-composited scrolling and that is tracked in crbug.com/753124.