mojo: Use TYPE_DEFAULT message-loop for browser-tests.
Use TYPE_DEFAULT message-loop for browser-tests, since that's what
mojo_runner uses by default. We would normally use TYPE_DEFAULT
message-loops for chrome --mash too, but keep using TYPE_UI for now,
since mojo:ui needs that, and it is not known whether mojo:ui will be
run at the type of the message-loop creation.
Also, remove references to MessagePumpMojo since that has been removed.
TBR=jam@ for base/threading/