Implement box-shadow for table section and row

A table section's normal box-shadow is painted before backgrounds of
colgroup, col and section are painted behind cells, then inset box-shadow.

A table row's normal box-shadow is painted before its background is
painted behind cells, then inset box-shadow.

BUG=94871

Review-Url: https://codereview.chromium.org/1963313002
Cr-Commit-Position: refs/heads/master@{#397041}
15 files changed