Experimental launcher: Switcher buttons are larger and centered.
Buttons are now 39x39, giving them bigger click/tap target zones,
spacing them further apart, and centering them within the switcher area.
The actual image sizes have not changed.
Moved the separator line up 1px to make room and to match the mocks.
Also added a new test of the exact dimensions of the app launcher, to
make sure we don't accidentally enlarge the window (which would have
been easy to accidentally do in this patch).
BUG=411775
Review URL: https://codereview.chromium.org/
581923002
Cr-Commit-Position: refs/heads/master@{#295934}