A source of latency when creating diff comments in large diffs is the
work needed to reflow the diff DOM to make room for the new comment.
This is particularly evident when adding comments to new files because
the diff is built as an addition group representing the entire file, so
the comment causes a reflow on every subsequent line.
This change optimizes this process in three ways.
* **Limit the size of ADD & REMOVE groups:** The diff processor will now
break an add or a remove chunk into a series of smaller chunks of the
same kind. This is controlled by the MAX_GROUP_SIZE constant. In this
way the number of nodes that need to be reflowed when a comment is
added to an add or remove group is limited to the number of subsequent
lines in that group plus the subsequent number of groups.
* **GPU optimize group in general:** Adds CSS properties to diff TBODY
elements (which correspond to groups, for the most part) that trigger
GPU acceleration when available.
* **Apply `table-layout: fixed;`** This style speeds up table reflow in
general.
Change-Id: Ie0e3665b7752fec67f7123cfae70ae99e6f67521
32b03fc76f