Store HTMLIFrameElement sandbox attr as TokenList

[jimrhoskins]

Fixes #3758

Store the sandbox attribute of HTMLIFrameElement as a TokenList
internally. Use .tokens() to iterate over the tokens instead of
splitting on the string value.

The external interface for sandbox remains a DOMString, which will need
to be fixed when DOMSettableTokenList is implemented (#1717).

[highfive]

Thanks for the pull request, and welcome! The Servo team is excited to review your changes, and you should hear from @metajack (or someone else) soon.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/3691

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[jdm]

r+, looks good!

[bors-servo]

saw approval from jdm
at jimrhoskins@e8036ac

[bors-servo]

merging jimrhoskins/servo/sandbox-tokenlist = e8036ac into auto

[bors-servo]

jimrhoskins/servo/sandbox-tokenlist = e8036ac merged ok, testing candidate = ebb702a

[bors-servo]

all tests pass:
success: http://build.servo.org/builders/linux1/builds/503
success: http://build.servo.org/builders/linux2/builds/501
success: http://build.servo.org/builders/android/builds/539
success: http://build.servo.org/builders/mac1/builds/720
success: http://build.servo.org/builders/mac2/builds/720

[bors-servo]

fast-forwarding master to auto = ebb702a