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: de016e52bd170d2d4f2344f9bf92d50478b649e0@70296 0039d316-1c4b-4281-b951-d872f2087c98

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