Bug 1420229 - Run formautofill xpcshell tests in headless mode;r=bdahl

This is needed because DOMParser is invoked on some fixture HTML that contains an
<svg> element (Walmart/Payment.html). When this happens **and** there is
a declaration for `color: MenuText` in a UA stylesheet (which is added in the next
changeset), there is a graphics crash. This is fixed in the new headless mode,
so this changeset forces the directory to run in it.

MozReview-Commit-ID: 9T8eFOW4xoz

--HG--
extra : rebase_source : d0cd013c2d1a980406c96a3fd6243ff9d2f33ed3

browser/extensions/formautofill/test/unit/xpcshell.ini

@@ -1,5 +1,6 @@
 [DEFAULT]
 firefox-appdir = browser
+headless = true
 head = head.js
 support-files =
   ../fixtures/**