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

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https://hg.mozilla.org/integration/gaia-central/rev/858a8bc1d64f
Author: James Lal <james@lightsofapollo.com>
Desc: Merge pull request #12401 from lightsofapollo/disable-browser-test-tmp

Temp disable browser test until bug 919584 is fixed

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https://hg.mozilla.org/integration/gaia-central/rev/0bdd954f5a21
Author: James Lal <james@lightsofapollo.com>
Desc: Temp disable browser test until bug 919584 is fixed