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

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https://hg.mozilla.org/integration/gaia-central/rev/b6c8b220ac30
Author: Yuren Ju <yurenju@gmail.com>
Desc: Merge pull request #20350 from yurenju/size-build

Bug 1020050 - [B2G] A check for gaia build size is needed r=testonly

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https://hg.mozilla.org/integration/gaia-central/rev/15bde1b08125
Author: Yuren Ju <yurenju@gmail.com>
Desc: Bug 1020050 - [B2G] A check for gaia build size is needed r=testonly

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https://hg.mozilla.org/integration/gaia-central/rev/c7fd174fe8f1
Author: steveck-chung <schung@mozilla.com>
Desc: Merge pull request #20337 from steveck-chung/Bug-1016897

Bug 1016897 - [Message] Previous saved draft will show up while entering edit new message view r=julienw

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https://hg.mozilla.org/integration/gaia-central/rev/53ad5206f924
Author: Steve Chung <schung@mozilla.com>
Desc: Bug 1016897 - [Message]Previous saved draft will show up while entering edit new message view
Bumping manifests a=b2g-bump