Set the right flags and attributes for InitEvent

[hmac]

Should fix #2189

This is my first PR for servo (and first time using Rust), so apologies if anything is horribly wrong.

[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/1333

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]

Went perfectly, thanks!

[bors-servo]

saw approval from Ms2ger
at https://github.com/hazz/servo/commit/7dbf69c22312821c1fa837d954fdf15148075588

[bors-servo]

merging hazz/servo/initevent = 7dbf69c into auto

[bors-servo]

hazz/servo/initevent = 7dbf69c merged ok, testing candidate = 73a0658

[bors-servo]

all tests pass:
success: http://servo-buildbot.pub.build.mozilla.org/builders/linux/builds/1259
success: http://servo-buildbot.pub.build.mozilla.org/builders/mac/builds/1213

[bors-servo]

fast-forwarding master to auto = 73a0658