CrOS Shelf: Change icon centering strategy

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}