Bumping gaia.json for 3 gaia-central revision(s)

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https://hg.mozilla.org/integration/gaia-central/rev/280fbd71cae2
Author: Alive.Kuo <alegnadise@gmail.com>
Desc: Merge pull request #10991 from jlebar/gaia-event-listener-leak

Bug 894081 - (Try to) fix a leak in Gaia by registering its mozbrowserer...r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/28d668d770e5
Author: Justin Lebar <justin.lebar@gmail.com>
Desc: Bug 894135 - Don't leak |homescreenFrame| from within window_manager.js's apploaded closure.

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https://hg.mozilla.org/integration/gaia-central/rev/b868997dbf0e
Author: Justin Lebar <justin.lebar@gmail.com>
Desc: Bug 894081 - (Try to) fix a leak in Gaia by registering its mozbrowsererror listener on the mozbrowser iframe itself, rather than on one of the iframe's parent nodes.