Generate diffing functions for generated subgroup BoxData in ComputedStyle

This patch generates the diffing functions for StyleBoxData which is now
a generated subgroup of ComputedStyle.

Please note that this patch doesn't generate the diffing for
VerticalAlign as it is accessed through both BoxData (when a length) and
a separate getter on ComputedStyle (when a keyword.) This will be
generated once we can generate diffs using functions.

Please note: No added callsite for ComputedStyleBase::ScrollAnchorDisablingPropertyChanged has been added
since it is already being called. We have just appended to that JSON
entry.

Diff: https://gist.github.com/5701f6d9a1be397677cbde5ad7033259/revisions

BUG=710938

Review-Url: https://codereview.chromium.org/2876353002
Cr-Commit-Position: refs/heads/master@{#471683}
2 files changed