Bug 933556 - Anchor fixed, left+right/top+bottom positioned content to center. r=roc

If a fixed position element has both top and bottom, or both left and right
positioning, set the fixed position anchor to the center of the corresponding
axis. This causes it to remain center-aligned when async zooming.
Bumping gaia.json for 5 gaia-central revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/38f10407c66d
Author: Zac <zcampbell@mozilla.com>
Desc: Merge pull request #13709 from viorelaioia/bug-938146

Bug 938146 - Put testvars skip into test_contacts_import_from_gmail

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https://hg.mozilla.org/integration/gaia-central/rev/485de82e210d
Author: Viorela Ioia <viorelaioia@gmail.com>
Desc: Bug 938146 - Put testvars skip into test_contacts_import_from_gmail

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https://hg.mozilla.org/integration/gaia-central/rev/8497221b23d4
Author: zacc <zcampbell@mozilla.com>
Desc: Bug 915138 - Python Change test_cards_view_with_three_apps.py to Hamachi failure only

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https://hg.mozilla.org/integration/gaia-central/rev/260273fb4965
Author: Bob Silverberg <bob.silverberg@gmail.com>
Desc: Merge pull request #13750 from zacc/bug_939114

Bug 939114 - Python test_camera_x tests failing

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https://hg.mozilla.org/integration/gaia-central/rev/9e2fdea9715a
Author: zacc <zcampbell@mozilla.com>
Desc: Bug 939114 - Python test_camera_x tests failing