Bug 1409580 - Skip test_bug511449.html in headless mode. r=jrmuizel

This could be enabled by adding support for SynthesizeNativeKeyEvent, but so
far this is the only test I've run into that needs this.

MozReview-Commit-ID: 9BWeHCc1fCi

--HG--
extra : rebase_source : c5b9fa25507816a988d072e290940a8d012af758

docshell/test/mochitest.ini

@@ -67,7 +67,7 @@ skip-if = true
 [test_bug475636.html]
 [test_bug509055.html]
 [test_bug511449.html]
-skip-if = toolkit != "cocoa"
+skip-if = toolkit != "cocoa" || headless # Headless: bug 1410525
 support-files = file_bug511449.html
 [test_bug529119-1.html]
 [test_bug529119-2.html]