Bug 1082629 - Collapsed icons should always be the same size. r=kgrandon

Columns on the homme-screen should not affect collapsed icon size.
Bug 1082629 - Add a marionette test to verify collapsed icon size. r=kgrandon

Add a marionette tests that makes sure collapsed icons are the same size,
regardless of the number of columns.
Merge pull request #25145 from Cwiiis/bug1082629-collapsed-icons-same-size

Bug 1082629 - Collapsed icons should always be the same size. r=kgrandon