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

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https://hg.mozilla.org/integration/gaia-central/rev/2860f076e014
Author: John Hu <johu@mozilla.com>
Desc: Merge pull request #15054 from huchengtw-moz/video/Bug_915039-no-string-change

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/4657e4f9e26f
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

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https://hg.mozilla.org/integration/gaia-central/rev/9ef07f058573
Author: John Hu <johu@mozilla.com>
Desc: Merge pull request #15053 from huchengtw-moz/video/Bug_915039_revert

Revert "Merge pull request #14745 from huchengtw-moz/video/Bug_915039_ad...

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https://hg.mozilla.org/integration/gaia-central/rev/0cbfbd2b1343
Author: John Hu <johu@mozilla.com>
Desc: Revert "Merge pull request #14745 from huchengtw-moz/video/Bug_915039_add_cancel_button"

This reverts commit a523c1ee4130a7883981924f8e68fb94be02428a, reversing
changes made to 83688a4f70774f1f96f4621f781024fcc925bce9.