Implement parsing and serialization of contain property

[upsuper]

Syntax: https://developer.mozilla.org/en-US/docs/Web/CSS/contain

This property should be added to https://github.com/servo/servo/blob/master/components/style/properties/longhand/box.mako.rs

Hacking guide for adding property: https://github.com/servo/servo/wiki/Property-hacking-guide

[highfive]

Hi! If you have any questions regarding this issue, feel free to make a comment here, or ask it in the #servo channel in IRC.

If you intend to work on this issue, then add @highfive: assign me to your comment, and I'll assign this to you. 😄

[hgallagher1993]

@highfive: assign me

[highfive]

Hey @hgallagher1993! Thanks for your interest in working on this issue. It's now assigned to you!

[KiChjang]

@hgallagher1993 You seem to have already claimed another issue, I'm unassigning this one until you finish the other one.

[jryans]

@highfive: assign me

[highfive]

Hey @jryans! Thanks for your interest in working on this issue. It's now assigned to you!

[hgallagher1993]

@KiChjang Ehh. . .can you see what issue? Because the last one (I remember) I was assigned I made the pull request for and the issue was closed

[KiChjang]

@hgallagher1993 Huh, sorry. This is really odd, I check my notifications daily and I was very sure that I saw you commenting on another issue, I just can't seem to search for it...

[jryans]

@hgallagher1993 Did you already make progress on this, or should I continue here?

[hgallagher1993]

@jryans I did absolutely nothing with it, so your pull request is fine to go ahead 😄