Fix printing unittests after revert of r230235

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
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=251102

[Alexei Svitkine (slow), 2014-02-13 19:42:49]

lgtm

[scottmg, 2014-02-13 19:43:07]

Committed patchset #1 manually as r251102 (tree was closed).

[Vitaly Buka (NO REVIEWS), 2014-02-13 19:51:15]

lgtm