Work towards #148028.

I'll likely start with DrawSolidBlueScreenTest as a minimal test and move from there.

This process may spawn additional features necessary for flutter_driver.

Some prototypes in the wild:

• https://github.com/johnmccutchan/flutter_test_native
• https://github.com/johnmccutchan/fgs

/cc @johnmccutchan @gaaclarke