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

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https://hg.mozilla.org/integration/gaia-central/rev/8308e3578c66
Author: Hubert Figuière <hub@figuiere.net>
Desc: Merge pull request #21724 from hfiguiere/master

Revert "Bug 981009 - Check that we have as many timings as we have runs for startup_test." r=me

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https://hg.mozilla.org/integration/gaia-central/rev/4869bc52301e
Author: Hubert Figuière <hub@figuiere.net>
Desc: Revert "Bug 981009 - Check that we have as many timings as we have runs for startup_test."

This reverts commit 93bdeec8365009434f694672d6db8f0eb25650c0.
Bumping manifests a=b2g-bump