Bumping gaia.json for 4 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/b5dd13046483
Author: Michael Henretty <michael.henretty@gmail.com>
Desc: Merge pull request #19809 from mikehenrty/bug-908300-withutils

Bug 908300 withutils

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https://hg.mozilla.org/integration/gaia-central/rev/9d6b0cb6b154
Author: Michael Henretty <michael.henretty@gmail.com>
Desc: Bug 908300 - use overflow and mutation observer to resize header font

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https://hg.mozilla.org/integration/gaia-central/rev/ea1de9dbbc38
Author: Yura Zenevich <yura.zenevich@gmail.com>
Desc: Merge pull request #19822 from yzen/bug-1018220

Bug 1018220 - adding missing aria attributes to make FTU screens more ac...

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https://hg.mozilla.org/integration/gaia-central/rev/88b0ddf7c659
Author: Yura Zenevich <yzenevich@mozilla.com>
Desc: Bug 1018220 - adding missing aria attributes to make FTU screens more accessible. r=fcampo
Bumping manifests a=b2g-bump