Support using MessagePumpForUIQt in standalone base::Threads

We currently assume that MessagePumpForUIQt will only be used on Qt's
GUI thread but OSX and Windows do have some cases where TYPE_UI is
used for non-UI threads. This currently causes asserts in debug on
OSX since the DNS thread quits prematurely.

Instead of overriding all those edge cases to use TYPE_DEFAULT,
properly support MessagePumpForUIQt::Run by using a QEventLoop.

Change-Id: Icdb65966867ca6fd3679c75a698007f63848babc
Reviewed-by: Andras Becsi <andras.becsi@digia.com>