Replace borrow with borrow_for_gc_trace in JSTraceable::trace (Issue #47...

[aweinstock314]

...78).

[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/4161

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.

[highfive]

Warning

• These commits modify unsafe code. Please review it carefully!

[jdm]

r+

[bors-servo]

saw approval from jdm
at aweinstock314@bcc27d9

[bors-servo]

merging aweinstock314/servo/master = bcc27d9 into auto

[bors-servo]

aweinstock314/servo/master = bcc27d9 merged ok, testing candidate = 417a932

[bors-servo]

all tests pass:
success: http://build.servo.org/builders/linux1/builds/909
success: http://build.servo.org/builders/linux2/builds/907
success: http://build.servo.org/builders/android/builds/945
success: http://build.servo.org/builders/mac1/builds/1124
success: http://build.servo.org/builders/mac2/builds/1124
success: http://build.servo.org/builders/gonk/builds/95

[bors-servo]

fast-forwarding master to auto = 417a932