Mac: Fix tab strip relayout bug.
A CL was recently introduced that prevented unnecessary tab-strip relayouts.
(https://codereview.chromium.org/
593223002/). That CL had a minor error where
it failed to relayout the tab strip when the left and right indentations
changed. This CL corrects that oversight.
This CL also refactors the browser_window_controller tab strip layout logic
into browser_window_layout. There is no intended behavioral change. The tab
strip layout logic now has test coverage.
BUG=415093
Review URL: https://codereview.chromium.org/
603243002
Cr-Commit-Position: refs/heads/master@{#296852}