[Gtk] RunLoop::run shuold run current thread's run loop.
https://bugs.webkit.org/show_bug.cgi?id=107887

Patch by Seulgi Kim <seulgikim@company100.net> on 2013-02-07
Reviewed by Martin Robinson.

Currently, RunLoop in Gtk can use just main thread's event loop.
But the other ports are implemented to use RunLoop in sub threads.

This patch makes RunLoop constructor create new context, not use default
context.
But in the main thread still uses default context to use main event loop
since there is some codes using glib directly (e.g. in
LayerTreeHostGtk::scheduleLayerFlush).

No new tests. There is no case that uses RunLoop in off the main thread
yet.

* platform/gtk/RunLoopGtk.cpp:
(WebCore::RunLoop::RunLoop):
(WebCore::RunLoop::run):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@142219 268f45cc-cd09-0410-ab3c-d52691b4dbfc