Bug 866450 Part 1: Fix rooting hazards under content/ and dom/ r=bz
Bug 866450 Part 2: Fix rooting hazards under content/ and dom/ r=bz
Bug 866450 Part 3: Fix rooting hazards under content/ and dom/ r=bz
Bug 866450 Part 4: Fix rooting hazards under content/ and dom/ r=bz
Bug 866450 Part 5: Fix rooting hazards under content/ and dom/ r=bz
Bug 866450 Part 6: Fix rooting hazards under content/ and dom/ r=bz
Bug 866450 Part 7: Root some additional JS::Values found during review r=bz