[ios] Fixes TableViewTextItem text color assignment.

TextItemColorBlack was being used as the default color even if
textColor was not being set. This was causing the item label
to skip the styler textColor for the label. This CL fixes that
problem and introduces some unit_tests to check this.

Bug: 865808
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Ic9e7d368d4acb5cab8f2585762c69363c1143749
Reviewed-on: https://chromium-review.googlesource.com/1145584
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Commit-Queue: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577003}
7 files changed