Bumping gaia.json for 4 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/539d09ea9898
Author: chirarobert <robert.chira@softvision.ro>
Desc: Merge pull request #24030 from zapion/Bug1066530

Bug 1066530 - tap_back_button doesn't work with new gaia-header

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https://hg.mozilla.org/integration/gaia-central/rev/f0b2877ad8b0
Author: Paul Zapion Yang <zapionator@gmail.com>
Desc: Bug 1066530 - tap_back_button doesn't work with new gaia-header

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https://hg.mozilla.org/integration/gaia-central/rev/94186e87fc94
Author: gitmai <mri@tid.es>
Desc: Merge pull request #23949 from gitmai/bug-1061858-CC-wifi-toggle-not-updated-by-race-condition

Bug 1061858 - [Cost Control][Usage app] Usage app disables Wifi tracking...
r=salva

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https://hg.mozilla.org/integration/gaia-central/rev/cd2b81e97fce
Author: mai <mri@tid.es>
Desc: Bug 1061858 - [Cost Control][Usage app] Usage app disables Wifi tracking after exiting/launching Usage app
Bumping manifests a=b2g-bump