[CSS Regions] Create helper method for checking if a box is fragmented into a specific region

• Component: CSS
• Status: RESOLVED INVALID

[Radu Stavila]

Now, the process is:
1. Declare start/end region pointers
2. Call getRegionRangeForBox on the flow thread to obtain the region of the box
3. Call regionInRange on the flow thread to check if the region of interest is in the box's range

This should be replaced with a helper method:
1. Call new method on flow thread isBoxFragmentedIntoRegion(region).