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

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https://hg.mozilla.org/integration/gaia-central/rev/3b9daa8b2380
Author: Zac <zcampbell@mozilla.com>
Desc: Merge pull request #14991 from zacc/bug_955626

Bug 955626 - Change wait in GaiaTest class to wait for AppWindowManager ...

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https://hg.mozilla.org/integration/gaia-central/rev/7a55339a30a5
Author: Zac Campbell <zcampbell@mozilla.com>
Desc: Bug 955626 - Change start_b2g wait in GaiaTest to wait for AppWindowManager to acknowledge the active frame instead of js event
Bumping manifests a=b2g-bump