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

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https://hg.mozilla.org/integration/gaia-central/rev/d9eb271e0b0e
Author: Min-Zhong "John" Lu <mnjul@users.noreply.github.com>
Desc: Merge pull request #20115 from mnjul/bug_1021479_lockscreen_visual_arrow_size

Bug 1021479 - [VsD Refresh] Lockscreen Visual Refresh: Slider arrow size; r=gweng
Merged even though Travis didn't pass; the failing Marionette tests were expected to fail, see https://groups.google.com/forum/#!topic/mozilla.dev.gaia/JQeF6PrVlrw

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https://hg.mozilla.org/integration/gaia-central/rev/f19a639e28f3
Author: John Lu [:mnjul] <jlu@mozilla.com>
Desc: Bug 1021479 - [VsD Refresh] Lockscreen Visual Refresh: Slider arrow size

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https://hg.mozilla.org/integration/gaia-central/rev/a282c5349b37
Author: Alive.Kuo <alegnadise@gmail.com>
Desc: Merge pull request #19973 from alivedise/bugzilla/1016334_2.0/homescreen-activity-window-home-event

Bug 1016334 - Do not change url of homescreen if there is front window, r=timdream

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https://hg.mozilla.org/integration/gaia-central/rev/1d49dea83d04
Author: Alive Kuo <alegnadise@gmail.com>
Desc: Bug 1016334 - Do not change url of homescreen if there is front window
Bumping manifests a=b2g-bump