Implement HTML{Anchor,Area,Link}Element.relList.

[achals]

#3994

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/3263

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[tetsuharuohzeki]

reviewed.

@jdm: do you have any points?

[jdm]

Yep, just a couple duplication/correctness things.

[achals]

Thanks for the feedback, updated.

[tetsuharuohzeki]

reviewed. please squashed. there would be no points.

[achals]

Thanks, done.

[jdm]

r+

[bors-servo]

saw approval from jdm
at achals@23ed1f7

[bors-servo]

merging achals/servo/master = 23ed1f7 into auto

[bors-servo]

achals/servo/master = 23ed1f7 merged ok, testing candidate = 6e19955

[bors-servo]

all tests pass:
success: http://build.servo.org/builders/linux1/builds/49
success: http://build.servo.org/builders/linux2/builds/49
success: http://build.servo.org/builders/android/builds/116
success: http://build.servo.org/builders/mac1/builds/266
success: http://build.servo.org/builders/mac2/builds/266

[bors-servo]

fast-forwarding master to auto = 6e19955