Modified comment in PlatformAppBrowserTest.WindowDotPrintWorks.

Modified comment in PlatformAppBrowserTest.WindowDotPrintWorks.

The PlatformAppBrowserTest.WindowDotPrintWorks test will only run
automatically for Chrome branded builds because it requires the PDF
plug-in, which isn't provided for Chromium builds. It can be run
manually for Chromium builds, however, and this change expands the
comment explaining how to do this on Linux to add an explanation of
how to do this on OS X.

TBR=asargent@chromium.org
BUG=none

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=222228

[commit-bot: I haz the power, 2013-09-09 20:50:31]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/dharcourt@chromium.org/23708018/1

[commit-bot: I haz the power, 2013-09-10 09:00:45]

Change committed as 222228