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

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https://hg.mozilla.org/integration/gaia-1_4/rev/f8ad9a110067
Author: David Flanagan <dflanagan@mozilla.com>
Desc: Merge pull request #20457 from davidflanagan/bug1015413

Bug 1015413: resolve large thumbnail flicker by uplifting bug 967089 to remove visibility monitor

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https://hg.mozilla.org/integration/gaia-1_4/rev/f9e09ce780f2
Author: John Hu <johu@mozilla.com>
Desc: Merge pull request #17837 from shamenchens/Bug967089-Remove-Visibility-Monitor-ImgSrc2

Bug 967089 - [gallery] remove visibility monitor code if it is no longer..., r=punam(cherry picked from commit 006cab98e7844de9adeb93f6faa8d33205c8b8f3)
Bumping manifests a=b2g-bump