Currently, accessibility of Chromium on desktop Linux is not really supported. However, there is a medium-term plan to solve this - please contact firstname.lastname@example.org if you'd like more information. Our goal is for all users to have an excellent experience, and we have a plan that will make everything work great while minimizing the extra effort required by Chromium developers.
The accessibility interface on Linux is the GNOME Accessibility ToolKit (ATK). After enabling accessibility support in the GNOME Preferences (in Ubuntu, check System -> Preferences -> Assistive Technologies), you can check Chrome's support using accerciser.