Bumping gaia.json for 6 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/dfeed4725146
Author: vingtetun <21@vingtetun.org>
Desc: Merge pull request #16697 from vingtetun/bug977209

Bug 977209 - Map a preference to a Gaia setting to enable dumping the la...

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https://hg.mozilla.org/integration/gaia-central/rev/5a82aedbe581
Author: Vivien Nicolas <21@vingtetun.org>
Desc: Bug 977209 - Map a preference to a Gaia setting to enable dumping the layers tree. r=fabrice

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https://hg.mozilla.org/integration/gaia-central/rev/5dcd2a328823
Author: vingtetun <21@vingtetun.org>
Desc: Merge pull request #16695 from vingtetun/bug977210

Bug 977210 - Factorize the html,body rules. r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/b0e4ed474468
Author: Vivien Nicolas <21@vingtetun.org>
Desc: Bug 977210 - Factorize the html,body rules. r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/6b01b8581385
Author: vingtetun <21@vingtetun.org>
Desc: Merge pull request #16696 from vingtetun/bug809031

Bug 809031 - OOP <iframe> content disappear if placed below 147px inside...

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https://hg.mozilla.org/integration/gaia-central/rev/1807b6f1e947
Author: Vivien Nicolas <21@vingtetun.org>
Desc: Bug 809031 - OOP <iframe> content disappear if placed below 147px inside the system. r=salva
Bumping manifests a=b2g-bump