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

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https://hg.mozilla.org/integration/gaia-central/rev/15a74cc58676
Author: John Hu <johu@mozilla.com>
Desc: Merge pull request #26965 from huchengtw-moz/bug-1114447-focus-context-menu

Bug 1114447 - [Stingray] switch focus to context menu while shown., r=rexboy

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https://hg.mozilla.org/integration/gaia-central/rev/23f8e0c803c2
Author: John Hu <johu@mozilla.com>
Desc: Bug 1114447 - [Stingray] switch focus to context menu while shown.
Bumping manifests a=b2g-bump