GTK: Allow file I/O in file dialog functions functions on the UI thread.

BUG=61611
TEST=Linux file dialogs don't trigger AssertIOAllowed DCHECKs.
Review URL: http://codereview.chromium.org/4321003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@64859 0039d316-1c4b-4281-b951-d872f2087c98