increasing the browser width and height on EC2 Test Agent

[pmeenan]

Currently the browsers are forced to run at 1024x768. The Chrome and Firefox agents can be told to change the browser size on a per-test basis using the setViewport command but there is no way to change it globally. The code also sets the desktop resolution on the EC2 agents to 1024x768 so there are a couple of changes that would need to be made to the code to make it possible (it is all doable, the code just doesn't support it right now).