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

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https://hg.mozilla.org/integration/gaia-central/rev/923a0923f49f
Author: RickyChien <ricky060709@gmail.com>
Desc: Merge pull request #18472 from RickyChien/bug-998757

Bug 998757 - Follow up of Bug 972699 - Sync the changes in js-test-agent and gaia r=@yurenju

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https://hg.mozilla.org/integration/gaia-central/rev/7390f903d05d
Author: Ricky Chien <ricky060709@gmail.com>
Desc: Bug 998757 - Follow up of Bug 972699 - Sync the changes in js-test-agent and gaia
Bumping manifests a=b2g-bump