GTK: tweak visibility calculation for bookmark bar items.
GTK: tweak visibility calculation for bookmark bar items.

Also vastly simplify the code for the common case where |extra_space| is 0.

Also fix a graphical imperfection that's been nagging me for a while where the bookmark bar looks kind of weird when you start dragging out of it. (the buttons jiggle around)

BUG=38186
TEST=manual

Review URL: http://codereview.chromium.org/1101003

git-svn-id: dd90618784b6a4b323ea0c23a071cb1c9e6f2ac7@42001 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