[iOS][Dark mode] Fix improper ordering of iOS 13 compile guards

Before, when compiling with iOS 12, but running with iOS 13, nothing
ended up running in these cases. Now, in this case, we fall back to the
iOS 12 logic.

Bug: 976681, 976666
Change-Id: If659155521ff2d8a13320aa2174e78e35476593a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1712875
Commit-Queue: Robbie Gibson <rkgibson@google.com>
Reviewed-by: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#679991}
2 files changed