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

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https://hg.mozilla.org/integration/gaia-central/rev/8bb872dc959b
Author: Timothy Guan-tin Chien <timdream@gmail.com>
Desc: Merge pull request #18101 from timdream/bug989974-try

Bug 989974 - Reenable system/HardwareButtons intermittent failing unit test, r=kgrandon

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https://hg.mozilla.org/integration/gaia-central/rev/b9d8e881aa83
Author: Timothy Guan-tin Chien <timdream@gmail.com>
Desc: Bug 989974 - Reenable system/HardwareButtons intermittent failing unit test
Bumping manifests a=b2g-bump