remoting: convert another unittest MessageLoop to MessageLoopForIO

remoting: convert another unittest MessageLoop to MessageLoopForIO

r126562 was incomplete and I didn't notice, probably because my muscle memory
builds Release by default and this only shows up in Debug builds.

This is exactly r126562 again, but for a different test.

TBR=wez
BUG=
TEST=

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=126664

[agl, 2012-03-14 17:03:36]

TBR because I had already landed a change in net/ which exposed this problem
(although it only affected Debug builds.)

[Wez, 2012-03-14 17:10:50]

lgtm