Mac: trivial clean up of tab strip layout code + unseeable layout fix.
Mac: trivial clean up of tab strip layout code + unseeable layout fix.

The placement of the new tab button was wrong when a placeholder is present ...
but in that case the new tab button wasn't shown.

BUG=none
TEST=none

Review URL: http://codereview.chromium.org/6036005

git-svn-id: dd90618784b6a4b323ea0c23a071cb1c9e6f2ac7@70296 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser