Bumping gaia.json for 3 gaia-central revision(s)

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https://hg.mozilla.org/integration/gaia-central/rev/21cca8ceda98
Author: GaryChen(pychen) <mmm198219@gmail.com>
Desc: Merge pull request #12120 from mpizza/Bug_908850_add_new_wallpaper

Bug 908850 - Add New Wallpapers for 1.2 for HVGA. r=yurenju

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https://hg.mozilla.org/integration/gaia-central/rev/d225519dc93b
Author: GaryChen(pychen) <mmm198219@gmail.com>
Desc: Bug 908850 - Add New Wallpapers for 1.2 for HVGA
    also fix Bug 908848 - Add New Wallpapers for 1.2 for WVGA

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https://hg.mozilla.org/integration/gaia-central/rev/7524547c649d
Author: b2gautomation <jonallengriffin@gmail.com>
Desc: Update gaia-ui-tests.json; Merge pull request #1349 from zacc/change_do_not_track_prefs

Change do not track tests and page object region

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "0b154cf0250488320f7569649eaaf69ed5a209a4", 
+    "revision": "21cca8ceda982e707ac6715e70c78e415b8e61f0", 
     "repo_path": "/integration/gaia-central"
 }