Initialize 'cancelable' to false in Event::new_inherited.

[mitchelljustin]

Fixes #3855.

I'm not sure if this is the right place to set cancelable. If changing the default value in Event::new_inherited is not satisfactory, it will be possible to set the Cell's value to false in Event::new itself.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/3060

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]

Replied on critic; please do make sure not to force-push as mentioned by @hoppipolla-critic-bot. Thanks!

[mitchelljustin]

@Ms2ger Alright, I've removed the .ini for Document-createEvent test since all tests pass now.

[Ms2ger]

Looking good!

[bors-servo]

saw approval from Ms2ger
at https://github.com/mvanderh/servo/commit/128f162b92f36cb2aedaf9c59dacbeadf588c12a

[bors-servo]

merging mvanderh/servo/master = 128f162 into auto

[bors-servo]

mvanderh/servo/master = 128f162 merged ok, testing candidate = 9b2dae5

[bors-servo]

all tests pass:
success: http://build.servo.org/builders/linux/builds/510
success: http://build.servo.org/builders/mac1/builds/55
success: http://build.servo.org/builders/mac2/builds/54

[bors-servo]

fast-forwarding master to auto = 9b2dae5