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

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https://hg.mozilla.org/integration/gaia-central/rev/8ff2792c4485
Author: Sam Foster <sam@sam-i-am.com>
Desc: Merge pull request #20164 from sfoster/bug-990035-video-tutorial

Bug 990035 - Modify phone-sized tutorial to show videsteps. r=borjasalguero

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https://hg.mozilla.org/integration/gaia-central/rev/399873247292
Author: Sam Foster <sfoster@mozilla.com>
Desc: Bug 990035 - Modify phone-sized tutorial to show vertical-scroll, notifications & sheets video. r=borjasalguero

* modify phone-layout tutorial config to use new video assets and tutorial strings
* placeholder video assets
* add optional callback to _setStep and handle image/video load errors
* hide overflow in tutorial step image/video container
* fix ui test for 3 steps
* remove orphaned tutorial images
* use .step-video-base to ensure video is height:100%
* Post-backout: Refactor tests and clarify expectation in event of config load failure
Bumping manifests a=b2g-bump