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

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https://hg.mozilla.org/integration/gaia-central/rev/28a75d32405f
Author: George <georgeiscoming@gmail.com>
Desc: Merge pull request #17379 from cctuan/977452

Bug 977452 - [keyboard refactor] Unit test for input_field.js

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https://hg.mozilla.org/integration/gaia-central/rev/3be4fa1e44bb
Author: cctuan <georgeiscoming@gmail.com>
Desc: Bug 977452 - [keyboard refactor] Unit test for input_field.js
Bumping manifests a=b2g-bump