Generate subgroup StyleSurroundData in ComputedStyle.

StyleSurroundData is a class that contains the values of the offset,
margin, padding and border properties. This patch generates this class
as a nested class inside ComputedStyle. We add an extra key in
CSSProperties.json5 called 'field_group' that specifies the name of the
group that the field is in. Fields with the same field_group are grouped
together into a nested class.

A few caveats:
- Causes perf regression because it doesn't exploit sharing of groups.
  The fix is in a dependent patch (to avoid bloating this CL). We
  will land the fix right after this patch is landed.
- We generate 'border' as 'storage_only', because it is a shorthand
  and not a CSS property.
- Subgroups are currently generated as nested classes of ComputedStyle,
  but this makes the code difficult to read. This may change in the
  future (such as putting subgroups in a ::detail namespace).
- We don't ASSERT_SIZE on subgroups yet.

Diff of generated files:


Cr-Commit-Position: refs/heads/master@{#467239}
17 files changed