Clean up StyleBackgroundData.

This patch:
- Removes its getters.
- Makes it inherit from RefCountedCopyable.
- Renames color_ to background_color

This is to set up for generating StyleBackgroundData.

BUG=628043

Review-Url: https://codereview.chromium.org/2854183003
Cr-Commit-Position: refs/heads/master@{#468905}
3 files changed