It currently has a ton of conditionals and large portions are specific to CHROMEOS. It should be refactored and cleaned up. I think we should do the following:
. Move DetachToBrowserTabDragControllerTest to the header. 
. Implementation of functions in DetachToBrowserTabDragControllerTest in tab_drag_controller_interactive_ui_test perform common functions and then call something like PlatformFoo.
. tab_drag_controller_interactive_uitest_common is used everywhere but chromeos and is the no-op implementations of the various PlatformFoo functions.
. Create tab_drag_controller_interactive_uitest_chromeos with all the chromeos specific bits.