Fix warnings in gonk port

[michaelwu]

This fixes a bunch of warnings in the gonk code. Not all of them, but we're only left with three now.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/4299

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[metajack]

r+

[larsbergstrom]

@bors: retry

[bors-servo]

saw approval from metajack
at michaelwu@67666c4

[bors-servo]

merging michaelwu/servo/fix-gonk-warnings = 67666c4 into auto

[bors-servo]

michaelwu/servo/fix-gonk-warnings = 67666c4 merged ok, testing candidate = c0dcb58

[bors-servo]

some tests failed:
failure: http://build.servo.org/builders/linux2/builds/1058

[bors-servo]

saw approval from metajack
at michaelwu@67666c4

[bors-servo]

merging michaelwu/servo/fix-gonk-warnings = 67666c4 into auto

[bors-servo]

michaelwu/servo/fix-gonk-warnings = 67666c4 merged ok, testing candidate = 8906d48

[bors-servo]

all tests pass:
success: http://build.servo.org/builders/linux1/builds/1059
success: http://build.servo.org/builders/linux2/builds/1059
success: http://build.servo.org/builders/android/builds/1093
success: http://build.servo.org/builders/mac1/builds/1275
success: http://build.servo.org/builders/mac2/builds/1275
success: http://build.servo.org/builders/gonk/builds/246

[bors-servo]

fast-forwarding master to auto = 8906d48

[larsbergstrom]

Retried because of intermittent failure logged as #5207