Implement DOMStringMap

[brunoabinader]

This is a sub-task for #2974.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/3075

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[jdm]

This needs to update the content test test_interfaces.html as well.

[bors-servo]

saw approval from jdm
at https://github.com/brunoabinader/servo/commit/a9d68474c93f635f3a6bb1a0dced3b373793f57d

[bors-servo]

merging brunoabinader/servo/dataset = a9d6847 into auto

[bors-servo]

brunoabinader/servo/dataset = a9d6847 merged ok, testing candidate = 46154fb

[bors-servo]

all tests pass:
success: http://build.servo.org/builders/linux/builds/558
success: http://build.servo.org/builders/mac1/builds/102
success: http://build.servo.org/builders/mac2/builds/101

[bors-servo]

fast-forwarding master to auto = 46154fb