Add firefox-specific directories to WPT skip list

Change-Id: I5f4004b03dee29cd8d4bbdd03b597caf916f140d
Reviewed-on: https://chromium-review.googlesource.com/1034295Reviewed-by: Robert Ma <robertma@chromium.org>
Commit-Queue: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554758}

third_party/WebKit/LayoutTests/W3CImportExpectations

@@ -122,6 +122,8 @@ external/wpt/dpub-aam [ Skip ]
 external/wpt/dpub-aria [ Skip ]
 external/wpt/fonts/math [ Skip ]
 external/wpt/html-longdesc [ Skip ]
+external/wpt/infrastructure/metadata/infrastructure/browsers/firefox [ Skip ]
+external/wpt/infrastructure/browsers/firefox [ Skip ]
 external/wpt/js [ Skip ]
 external/wpt/mathml [ Skip ]
 external/wpt/old-tests [ Skip ]