Stop including the element during Element.getElementsByClassName.

[achals]

#3995

This is my first PR, so please let me know if I'm doing something wrong!

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/3209

This is an external review system which you may optionally use for the code review of your pull request.

In order to help critic track your changes, please do not make in-place history rewrites (e.g. via git rebase -i or git commit --amend) when updating this pull request.

[Ms2ger]

@achals: Thanks! I commented about one small nit on critic; please push an additional commit to fix that.

[achals]

@Ms2ger Thanks for the feedback, updated

[Ms2ger]

@achals: thank you! Please squash the two commits together so I can merge.

[achals]

@Ms2ger Squashed, can you please check if things look okay?

[Ms2ger]

r+

[bors-servo]

saw approval from Ms2ger
at achals@712b3d0

[bors-servo]

merging achals/servo/master = 712b3d0 into auto

[bors-servo]

achals/servo/master = 712b3d0 merged ok, testing candidate = 8cecb03

[bors-servo]

all tests pass:
success: http://build.servo.org/builders/linux1/builds/8
success: http://build.servo.org/builders/linux2/builds/8
success: http://build.servo.org/builders/android/builds/75
success: http://build.servo.org/builders/mac1/builds/225
success: http://build.servo.org/builders/mac2/builds/225

[bors-servo]

fast-forwarding master to auto = 8cecb03