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This patch adds the fieldwise_diff macro and then uses it to generate
the diff functions on the groups that have been generated so far
(StyleSurroundData) in ComputedStyleBase.

Please note that it can only be used for memebers of those groups too
that have already been generated. This is why the diffing for
BorderData has been left to a later CL.

Diff: https://gist.github.com/nainar/04f49165c4cb5ecb30371fbde1491ddf/revisions

BUG=710938

Review-Url: https://codereview.chromium.org/2858863002
Cr-Commit-Position: refs/heads/master@{#469286}