Constant style::parallel::WORK_UNIT_MAX

pub const WORK_UNIT_MAX: usize = 16

The maximum number of child nodes that we will process as a single unit.

Larger values will increase style sharing cache hits and general DOM locality at the expense of decreased opportunities for parallelism. There are some measurements in https://bugzilla.mozilla.org/show_bug.cgi?id=1385982#c11 and comments 12 and 13 that investigate some slightly different values for the work unit size. If the size is significantly increased, make sure to adjust the condition for kicking off a new work unit in top_down_dom, because otherwise we're likely to end up doing too much work serially. For example, the condition there could become some fraction of WORK_UNIT_MAX instead of WORK_UNIT_MAX.