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

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https://hg.mozilla.org/integration/gaia-central/rev/421494f737e3
Author: Alive.Kuo <alegnadise@gmail.com>
Desc: Merge pull request #11861 from alivedise/bugzilla/909255/window-disposition-activity

Bug 909255 - Remeber window disposition activity caller, r=timdream

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https://hg.mozilla.org/integration/gaia-central/rev/508adc5d9943
Author: Alive Kuo <alegnadise@gmail.com>
Desc: Bug 909255 - Remeber window disposition activity caller

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https://hg.mozilla.org/integration/gaia-central/rev/d62138d349e5
Author: Cristian Rodriguez <crdlc@tid.es>
Desc: Merge pull request #11903 from crdlc/bug-911875

Bug 911875 - [Homescreen] When user select icon and press home key, selected color is shown on icon

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https://hg.mozilla.org/integration/gaia-central/rev/74c560df603b
Author: crdlc <crdlc@tid.es>
Desc: Bug 911875 - [Homescreen] When user select icon and press home key, selected color is shown on icon