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

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https://hg.mozilla.org/integration/gaia-central/rev/6a554d39cbdf
Author: John Hu <johu@mozilla.com>
Desc: Merge pull request #14745 from huchengtw-moz/video/Bug_915039_add_cancel_button

Bug 915039 - [Video] There is no easy way to navigate back to the activi..., r=djf

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https://hg.mozilla.org/integration/gaia-central/rev/b28c38dde7d6
Author: John Hu <johu@mozilla.com>
Desc: Bug 915039 - [Video] There is no easy way to navigate back to the activity caller if the library is empty