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

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https://hg.mozilla.org/integration/gaia-central/rev/28bd1a978f4d
Author: John Hu <johu@mozilla.com>
Desc: Merge pull request #12603 from huchengtw-moz/video/Bug_915911_read_title_from_metadata

Bug 915911 - [Video] Saving a video from web will not retain the video n..., r=djf

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https://hg.mozilla.org/integration/gaia-central/rev/28fd995b8dcc
Author: John Hu <johu@mozilla.com>
Desc: Bug 915911 - [Video] Saving a video from web will not retain the video name, instead says "Videoplayback"
Bug 855208: Allow one newly created content channel to play in the background. r=mchen