Fix the crashes in interactive_ui_tests on Linux.
The cause is really subtle. When I added a db_thread() in BrowserMain, that changed the order that threads were created at. Since interactive ui tests don't run atexitmanager (I filed a bug to make them do), this cascaded into file_posix.cc's InFlightIO singelton caching the first IO MessageLoop pointer. By fluke, previously each IO ML would have the exact same pointer value (must be a unique size for the allocator). My change modified the construction order, so the second run would have a different ChromeThread (file) get the previous IO thread's ML pointer. That led to the asserts. I have added code to start the threads in a predictable manner for now.
TEST=interactive_ui_tests in Linux stop crashing