Disable the ChromeFrame Firefox tests on Windows 7 due to a bug in Firefox causing it to crash

on startup after the machine has been running for a while. For the curious the crash occurs
in the tab preview code in Firefox which only executes on Windows 7 and executes even if
we disable tab preview.

BUG=none
TEST=none

Review URL: http://codereview.chromium.org/5745008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@69218 0039d316-1c4b-4281-b951-d872f2087c98