[TableFragmentation] Paint section/row backgrounds in cells correctly.

This correctly calculates the part-rect when fragmentation is present.
We use the OffsetInStitchedFragments to currently get the part size,
and offset, and use that to correctly paint the table section/row
backgrounds in the cell.

There shouldn't be any significant performance delta due to this patch.

Bug: 1078927
Change-Id: I05eb09e9221005c4da9404508317c1bf99687cce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3632829
Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1001100}