Add GNOME 3 style scrollbars to Chrome. (Install adwaita-chrome-scrollbars-small for smaller scrollbars.)
Make Chrome's scrollbars look like GTK+3 applications (as seen in GNOME 3), thanks to the magic of CSS.
You'll also want to install the Adwaita Chrome theme too:
Version 1.2 splits the extension into two different ones. The standard now has default-sized scrollbars, and there is now a small variant with the original, smaller size scrollbars available too.
The small variant is at: https://chrome.google.com/webstore/detail/ipieahdlgpmkipjokemifjnjmnhkdhdh
Version 1.1 disables custom scrollbars in iframes, to work around Chrome issue 57292. This notably fixes GMail (and any other site relying on scrollbars in iframes). More information on the bug is available at: http://code.google.com/p/chromium/issues/detail?id=57292