R1:1e9492f8c2ad

Show coverage information in Differential

Summary:
Render coverage information in the right gutter, if available.

We could render some kind of summary report deal too but this seems like a good
start.

Test Plan:
- Looked at diffs with coverage.
- Looked at diffs without coverage.
- Used inline comments, diff-of-diff, "show more", "show entire file", "show
generated file", "undo". Nothing seemed disrupted by the addition of a 5th
column.

Reviewers: btrahan, tuomaspelkonen, jungejason

Reviewed By…