Bumping gaia.json for 2 gaia-central revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/6489f84d8564
Author: gasolin <gasolin@gmail.com>
Desc: Merge pull request #13051 from gasolin/issue-917367

Bug 917367 - No notification visible indicator implemented for when getU..., r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/2d636b01ef55
Author: gasolin <gasolin@gmail.com>
Desc: Bug 917367 - No notification visible indicator implemented for when getUserMedia audio is active