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

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https://hg.mozilla.org/integration/gaia-central/rev/4ea7dd7784d2
Author: steveck-chung <schung@mozilla.com>
Desc: Merge pull request #19719 from steveck-chung/bug-963109

Bug 963109 - [Messages] Multi-recipient participants view should show back button instead of cross one. r=azasypkin

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https://hg.mozilla.org/integration/gaia-central/rev/16e8be2c0e6a
Author: Steve Chung <schung@mozilla.com>
Desc: Bug 963109 - [Messages] Multi-recipient participants view should show back button instead of cross one
Bumping manifests a=b2g-bump