Store border-image-* on SurroundData in ComputedStyle

This patch moves the storage of border-image-* away from BorderStyle.
We instead store them on the generated SurroundData group on
ComputedStyle instead.

We generate a BorderValue to pass to Layout in
ComputedStyle from all the border information stored
there.

Diff: https://gist.github.com/a0eeb1d1383e4b502604b449867f9619/revisions

BUG=628043

Review-Url: https://codereview.chromium.org/2891003002
Cr-Commit-Position: refs/heads/master@{#473173}
8 files changed