Add regression test to verify array with packed, holey and
dictionary elements.
Change ToUnicodeStringArray to return vector<UnicodeString>
instead allocate raw UnicodeString before calling.
Simplify ToUnicodeStringArray to loop only once.

Bug: chromium:903566
Change-Id: I7ad74179be97d3cf929d2949384dbaa8b66a9a02
Reviewed-on: https://chromium-review.googlesource.com/c/1328642
Commit-Queue: Frank Tang <ftang@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57521}