Remove references to surround_data_ in ComputedStyle.

To allow changes to where fields are stored, ComputedStyle code should
not refer directly to a group, as that code will break when we change
groups. This patch removes references to surround_data_ in ComputedStyle,
replacing with generated or handwritten getters.

References in the cpp file have already been removed.

BUG=710938

Review-Url: https://codereview.chromium.org/2887433005
Cr-Commit-Position: refs/heads/master@{#472322}
1 file changed