Bumping gaia.json for 4 gaia-central revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/a4ed36c4ee84
Author: gasolin <gasolin@gmail.com>
Desc: Merge pull request #13790 from MBRSL/bug-938952-fullscreen

Bug 938952 - [UITest] provide a way to exit fullscreen mode, r=gasolin

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https://hg.mozilla.org/integration/gaia-central/rev/9a08fc6cb68e
Author: Tom Jao <tjao@mozilla.com>
Desc: Bug 938952 - [UITest] provide a way to exit fullscreen mode

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https://hg.mozilla.org/integration/gaia-central/rev/1f40468dc7fd
Author: gasolin <gasolin@gmail.com>
Desc: Merge pull request #13866 from MBRSL/bug-939016-CSS

Bug 939016 - [UITest] Move css out of HTML, r=gasolin

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https://hg.mozilla.org/integration/gaia-central/rev/bc1b421332e0
Author: Tom Jao <tjao@mozilla.com>
Desc: Bug 939016 - [UITest] Move css out of HTML