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

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https://hg.mozilla.org/integration/gaia-central/rev/8bca26da73b4
Author: Alive.Kuo <alegnadise@gmail.com>
Desc: Merge pull request #17016 from alivedise/bugzilla/977934_master_v4/render-activity-inside-caller

Bug 977934 - Render activity frame inside its caller, r=etienne

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https://hg.mozilla.org/integration/gaia-central/rev/0e31d1af1e9d
Author: Alive Kuo <alegnadise@gmail.com>
Desc: Bug 977934 - Render activity frame inside its caller

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https://hg.mozilla.org/integration/gaia-central/rev/2adcd9949b1b
Author: Dominic Kuo <dkuo@mozilla.com>
Desc: Merge pull request #16964 from dominickuo/bug-973156-new-2

Bug 973156 - [Music] Send mozinterruptbegin and mozinterruptend to system for show/hide the media playback widget, r=jimporter

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https://hg.mozilla.org/integration/gaia-central/rev/722535f2aa0c
Author: Dominic Kuo <dkuo@mozilla.com>
Desc: Bug 973156 - [Music] Send mozinterruptbegin and mozinterruptend to system for show/hide the media playback widget
Bumping manifests a=b2g-bump