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

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https://hg.mozilla.org/integration/gaia-central/rev/64ce5f9516f3
Author: Ian Liu <iliu@mozilla.com>
Desc: Merge pull request #16282 from ian-liu/system/bug964653_system_dialog_refactor

Bug 964653 - (system-dialog) [Window Management] Implement System Dialog Window, r=@alivedise

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https://hg.mozilla.org/integration/gaia-central/rev/766cdb825bd5
Author: ian-liu <iliu@mozilla.com>
Desc: Bug 964653 - (system-dialog) [Window Management] Implement System Dialog Window
Bumping manifests a=b2g-bump