Bumping gaia.json for 1 gaia-1_3 revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-1_3/rev/a862e6512c07
Author: Mason Chang <mason@masonchang.com>
Desc: Merge pull request #15367 from changm/bug950250

Bug 950250 Make background scroll with content r=arthurcc