test-unboxed-doubles/WriteBarrierObjectShiftFieldsRight recently started
failing on arm64-nosnapshot builds due to a broken CHECK.

# Fatal error in ../../test/cctest/test-unboxed-doubles.cc, line 1417
# Check failed: heap->InNewSpace(*obj_value).

It expects the result of Factory::NewJSArray() to be in new
space; but NewJSArray encapsulates two allocations so the return value can
actually be in old space. Fix it by ensuring only one allocation occurs.

BUG=v8:5339

Review-Url: https://codereview.chromium.org/2759433002
Cr-Commit-Position: refs/heads/master@{#43886}