Bumping gaia.json for 6 gaia-central revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/62ffee1a7ebd
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Merge pull request #13780 from pivanov/bug-939152

Bug 939152 - [Action Icons - Shared] Remove shadows from action icons

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https://hg.mozilla.org/integration/gaia-central/rev/ed664bf6a78f
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Bug 939152 - [Action Icons - Shared] Remove shadows from action icons

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https://hg.mozilla.org/integration/gaia-central/rev/c4c122423a56
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Merge pull request #13808 from pivanov/bug-939224

Bug 939224 - [Action Icons - Notification Tray] Remove shadows from action icons

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https://hg.mozilla.org/integration/gaia-central/rev/519916294987
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Bug 939224 - [Action Icons - Notification Tray] Remove shadows from action icons

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https://hg.mozilla.org/integration/gaia-central/rev/5be6169c4d88
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Merge pull request #13804 from pivanov/bug-939232

Bug 939232 - [Action Icons - Lockscreen] Remove shadows from action icons

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https://hg.mozilla.org/integration/gaia-central/rev/39faff288e76
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Bug 939232 - [Action Icons - Lockscreen] Remove shadows from action icons