In pages with large diffs, creating the first comment thread can be
slow. With this change, the GR-DIFF-BUILDER makes a hidden thread and
attaches it to the page, then removes it. This causes a much faster
render when the user creates a comment.
Below are some performance numbers based around creating comments in the
large reference diff that is linked in the issue. The measurements are
made on a MacBook Pro with an Intel Core i7, so the difference is all
the more pronounced on slower machines.
| Before | After
--------------------+---------+-------
First Comment | ~820 ms | ~95 ms
--------------------+---------+-------
Subsequent Comments | ~50 ms | ~50 ms
Bug: Issue 4335
Change-Id: I649474320afce1b7daa0ad47753bb11223cc305b