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The main change here is that icons are first centered on the whole
screen, then, when space becomes tighter, they are centered over the
available area on the shelf.

A few changes to make this work properly:

* Fix an issue in |GetDimensionOfCenteredShelfItems| which was taking
  into account all items, even the ones within overflow, making the
  calculations incorrect.
* Another issue was that the code calculating ideal bounds would
  assume control buttons (app list, back, overflow) were the same size
  as app buttons (56) while they're actually a little smaller (40).
  Giving control buttons their "real" size in the layout code allows
  for the removal of some overrides at each button class level.
* Split out a small chunk from |CalculateIdealBounds| which is getting
  too long. More of that to come in future changes.
* Make the special spacing between the app list button and the first
  app icon also there on the other side for more symmetry.

Also add some extensive test coverage.

Bug: 891080, 933291
Change-Id: If4bb69aa6e182cd29c02d467039b5d839d09f493
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1512299
Commit-Queue: Manu Cornet <manucornet@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642245}