Disable ExternallyConnectableMessagingTest.BackgroundPageClosesOnMessageReceipt test.

http://test-results.appspot.com/dashboards/flakiness_dashboard.html#group=%40ToT%20Chromium&testType=browser_tests&tests=ExternallyConnectableMessagingTest.BackgroundPageClosesOnMessageReceipt

TBR=juanlang@google.com
BUG=297866
TEST=none

Review URL: https://codereview.chromium.org/24453003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225091 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225091