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

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https://hg.mozilla.org/integration/gaia-central/rev/c54593431494
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Merge pull request #20807 from etiennesegonzac/bug-1026240-will-change-overuse

Bug 1026240 - Removing unnecessary will-change uses in the system app. r=21

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https://hg.mozilla.org/integration/gaia-central/rev/4ea70716a771
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Bug 1026240 - Removing unnecessary will-change uses in the system app.
Bumping manifests a=b2g-bump