overlay.set_view_area(w, h) results in doubled size with headless chrome using a devicePixelRatio=2 display

[kswang1029]

When we call overlay.set_view_area(w, h) to create a plot with a width of w pixels and a height of h pixels, we will get a png with 2w x 2h pixels if we use a Retina display (ie Mac laptop). To fix this I will need to specify a chrome option --force-device-scale-factor=1 when creating a session such as

session = Session.create(Chrome(headless=True, options=['--use-gl=angle', '--force-device-scale-factor=1','--force-color-profile=srgb']), url_without_token, debug_no_auth=True)

Maybe we should set the flag as default to make it consistent?