Fix printing unittests after revert of r230235

r251066 reverted the backgrounding of the print dialog.

Before the revert

http://src.chromium.org/viewvc/chrome/trunk/src/printing/printing_context_win_unittest.cc?r1=232466&r2=232611

assumed they were going to be async, so added RunLoop. Now that the
dialog is synchronous again, remove that code which is causing the
MessageLoop to complain.

TBR=vitalybuka@chromium.org
R=asvitkine@chromium.org
BUG=180997,342697

Review URL: https://codereview.chromium.org/164483002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251102 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251102