Bug 945222 - Add test harness for running W3C web-platform-tests.

This adds a new test harness, and an associated
mach command |mach web-platform-tests|.

At present only desktop firefox is supported. Tests
are run in parallel using multiple firefox instances.

It also adds a copy of the web platform tests and a
(incomplete) script for updating them.

--HG--
extra : rebase_source : 2c51b7393166df4ebea997e978c133284a62d49b
Bug 945222 - Import of web-platform-tests repository
* * *
Bug 0 - Update web-platform-tests to revision cc96f051bc10479cd5b99f7a04a604b145ddece8

--HG--
extra : rebase_source : 52a2d0d483570569ab8d68dc5c045f73cd03030e
Bug 945222 - Add some expected data for web-platform-tests
* * *
Bug 0 - Update web-platform-tests expected data to revision cc96f051bc10479cd5b99f7a04a604b145ddece8