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

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https://hg.mozilla.org/integration/gaia-central/rev/7939d468b2b9
Author: George <georgeiscoming@gmail.com>
Desc: Merge pull request #24642 from cctuan/1049439

Bug 1049439 - change appWindowManager from singleton to class

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https://hg.mozilla.org/integration/gaia-central/rev/2753f85fef8c
Author: cctuan <georgeiscoming@gmail.com>
Desc: Bug 1049439 - change appWindowManager from singleton to class
Bumping manifests a=b2g-bump