Make support for configuring AdhocBrowserPool to always load google chrome extension into all instances

[wzin]

Overview:

It makes sense to be able to configure a browser pool (for example the touchscreen pool) to always load a chrome extension into every chrome instance that it shepherds.

Requirements:

• make a ROS param static_extensions that will provide a list of names of chrome extensions that will be always loaded into every chrome instance managed by the pool
• write obsessive adhocbrowser tests for that

Testing scenario

• configure a pool with an extension that should be loaded
• test it accessing the debugging port and checking if it's there