Permit site administrators to configure a URL for users to switch the
current user identity. CacheBasedWebSession automatically invalidates
the old session (and the cookie and XSRF token) during the switch.
For DEVELOPMENT_BECOME_ANY_ACCOUNT this is a faster way to jump to
another test user as it avoids needing to logout first, saving one
click and a reload of the web UI.
For OPENID the user can select another OpenID user quickly using the
login dialog. This can be useful if someone has two personas, such as
a personal identity used with open source work and another identity
used for day-job or administrative activities.
On gerrit-review the configuration setting will be set to facilitate
switching between a user's Google Accounts.
Change-Id: I8abd58c06e74f96fef5430324e1c24a787823d35
e0cafe4ecd