Bumping gaia.json for 3 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/a135e65a8a96
Author: vingtetun <21@vingtetun.org>
Desc: Merge pull request #20242 from vingtetun/devtools.system

Devtools.system

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https://hg.mozilla.org/integration/gaia-central/rev/3c0b0e090e49
Author: Vivien Nicolas <vnicolas@mozilla.com>
Desc: Bug 1022724 - Display a devtool overlay for the system app. r=janx

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https://hg.mozilla.org/integration/gaia-central/rev/a83c8d9fba8c
Author: Vivien Nicolas <vnicolas@mozilla.com>
Desc: Bug 971008 - Don't cause System App reflow noise in the Developer HUD. r=janx
Bumping manifests a=b2g-bump