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

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https://hg.mozilla.org/integration/gaia-central/rev/900b43e1e6cc
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Merge pull request #18890 from etiennesegonzac/bug-986062-edge-reflow-test

Bug 986062 - Adding a test making sure we don't do any reflows while using the edge gestures.
r=vingtetun

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https://hg.mozilla.org/integration/gaia-central/rev/74ee42ce433c
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Bug 986062 - Adding a test making sure we don't do any reflows while using the edge gestures.

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https://hg.mozilla.org/integration/gaia-central/rev/fe9772a708f9
Author: Carsten Book <tomcat@mozilla.com>
Desc: Merge pull request #19080 from kamituel/Bug_1007237

Bug 1007237 - encodeHandoverRequest() unit tests and improvements. r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/c1f993d9ff35
Author: Kamil Leszczuk <kamituel@gmail.com>
Desc: encodeHandoverRequest() tests and fixes.

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https://hg.mozilla.org/integration/gaia-central/rev/2c604aaa8523
Author: Carsten Book <tomcat@mozilla.com>
Desc: Merge pull request #18866 from yzen/bug-995258

Bug 995258 - fixed rocketbar screen reader activation and under-backdrop.... r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/2d7f8d34df50
Author: Yura Zenevich <yzenevich@mozilla.com>
Desc: Bug 995258 - fixed rocketbar screen reader activation and under-backdrop visibility. r=alive
Bumping manifests a=b2g-bump