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

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https://hg.mozilla.org/integration/gaia-central/rev/acad84391b6f
Author: gasolin <gasolin@gmail.com>
Desc: Merge pull request #13936 from MBRSL/bug-929313-CSS

Bug 929313 - [UITest] CSS dimension test, r=gasolin

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https://hg.mozilla.org/integration/gaia-central/rev/7c252933a8d5
Author: Tom Jao <tjao@mozilla.com>
Desc: Bug 929313 - [UITest] CSS dimension test