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

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https://hg.mozilla.org/integration/gaia-central/rev/8669bf55fc54
Author: gasolin <gasolin@gmail.com>
Desc: Merge pull request #15236 from MBRSL/bug-959027

Bug 959027 - [UITest] Fix window.close() button, r=gasolin

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https://hg.mozilla.org/integration/gaia-central/rev/3afe75ed1c9f
Author: Tom Jao <tjao@mozilla.com>
Desc: Bug 959027 - [UITest] Fix window.close() button