Initialize Event::bubbles to false

[campadrenalin]

Fixes #2183.

[highfive]

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

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/1387

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[Ms2ger]

r+

[bors-servo]

saw approval from Ms2ger
at campadrenalin@6312ff3

[bors-servo]

merging campadrenalin/servo/initialize-event-bubbling-to-false = 6312ff3 into auto

[bors-servo]

campadrenalin/servo/initialize-event-bubbling-to-false = 6312ff3 merged ok, testing candidate = b6ed246

[bors-servo]

all tests pass:
success: http://servo-buildbot.pub.build.mozilla.org/builders/linux/builds/1288
success: http://servo-buildbot.pub.build.mozilla.org/builders/mac/builds/1242

[bors-servo]

fast-forwarding master to auto = b6ed246