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Implement code using updateServiceGroup. Specifically create a group for
service connections that have low rank. Currently the cutoff is at
"invisible main frame" and below.

Now implementation uses own sort implementation by taking advantage that
updates happen to a single connection at a time only. NB the simplified
sort is no longer stable, and specifically pulls up connection's rank
whenever it's updated.

Algorithm for assigning importance in group is to pick a value between
importance of connection to the left and to the right (if any), and
reshuffle the entire list of there is not enough room left. There is
a bias of assigning closer to the right; this is an optimization to
reduce shuffle since by far the most common operation is moving a
connection to be the most important (ie with low importance value)
in the group.

Add a feature to enable this so we can experiment when there are
enough users.

Bug: 946216
Change-Id: I2ea4437b9b27eb205b7c2e51344f2af387938232
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1597071Reviewed-by: ssid <ssid@chromium.org>
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Commit-Queue: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659436}