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

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https://hg.mozilla.org/integration/gaia-central/rev/710aa8878ba3
Author: Jim Porter <jporter@mozilla.com>
Desc: Merge pull request #21071 from jimporter/music-perf-events

Bug 837675 - [music] Implement new startup loading events

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https://hg.mozilla.org/integration/gaia-central/rev/26301c162dce
Author: Jim Porter <jporter@mozilla.com>
Desc: Bug 837675 - [music] Implement new startup loading events
Bumping manifests a=b2g-bump