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This makes the bubbles ten pixels shorter and implements
the Bubble class's fade-in/fade-out behavior for GTK+ (which
also e.g. makes bookmark bubbles now fade away when you
click their "Done" buttons or "Remove" links, like on other
platforms).

BUG=chromium-os:8473
TEST=manual: configured the setting bubbles to fade in and out and checked that they looked correct

Review URL: http://codereview.chromium.org/7529038

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@96004 0039d316-1c4b-4281-b951-d872f2087c98