mac: Tweak minimum tab width to look good on retina displays too

mac: Tweak minimum tab width to look good on retina displays too

BUG=168820
TEST=open lots and lots of tabs. The close button stays centered in
the selected tab, and the background tabs don't become so small that
they look clipped. If too many tabs are opened, they just wander
out of the window to the right, like previously -- this time do this
on a retina display.

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=175857

[Nico, 2013-01-09 18:27:25]

I hope reviewing these stimulates you intellectually just as much writing them
stimulates me.

[Nico, 2013-01-09 19:52:49]

sail seems to not come in before noon these days.

[Robert Sesek, 2013-01-09 19:53:04]

lgtm