Move body of ScriptTask::handle_mouse_move_event into a method on Document (fixes #5032).

[JIoJIaJIu]

No description provided.

[highfive]

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

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/4113

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[jdm]

Looks good! Just needs a squash and rebase before we merge it :)

[Ms2ger]

r=jdm

[Ms2ger]

fixes #5032

[bors-servo]

saw approval from jdm
at JIoJIaJIu@94e7f89

[bors-servo]

merging JIoJIaJIu/servo/handle_mouse_event = 94e7f89 into auto

[bors-servo]

JIoJIaJIu/servo/handle_mouse_event = 94e7f89 merged ok, testing candidate = 75060f4

[bors-servo]

all tests pass:
success: http://build.servo.org/builders/linux1/builds/884
success: http://build.servo.org/builders/linux2/builds/882
success: http://build.servo.org/builders/android/builds/918
success: http://build.servo.org/builders/mac1/builds/1099
success: http://build.servo.org/builders/mac2/builds/1099
success: http://build.servo.org/builders/gonk/builds/70

[bors-servo]

fast-forwarding master to auto = 75060f4