Move StyleMultiColData::RuleWidth to ComputedStyle.

When we generate StyleMultiColData, we would no longer have the
RuleWidth method because it is handwritten on StyleMultiColData.
This patch moves RuleWidth to be inline in ComputedStyle so that we
can generate StyleMultiColData.

BUG=628043

Review-Url: https://codereview.chromium.org/2916843002
Cr-Commit-Position: refs/heads/master@{#476560}
2 files changed