It is easy to forget to set this flag on macOS, where doing so causes
many tests to fail. So let's just set it via code along with other
options to make it a bit more foolproof.
The headless-browser source is getting a bit unwieldy. The ordering of
class and method definitions is fragile; e.g. the application and web
view classes each require full definitions of each other. So it has
reached the point where it makes sense to give headless-browser some
better file structure.
To prepare for that, this patch simply moves its source to live along-
side the other browser chromes. This location is a bit better prepared
for creating more files, as the Utilities folder doesn't even have its
own CMakeLists.txt.
