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

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https://hg.mozilla.org/integration/gaia-central/rev/46f6f5683923
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Merge pull request #18453 from daleharvey/997521-redux

Bug 997521 - Add prepopulated top sites and history. r=kgrandon

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https://hg.mozilla.org/integration/gaia-central/rev/579b516192e0
Author: Dale Harvey <dale@arandomurl.com>
Desc: Bug 997521 - Add prepopulated top sites and history. r=kgrandon
Bumping manifests a=b2g-bump