[iOS] [iOS13] Fix assumptions in UILabelAttributeUtilsTest

This CL fixes an assumption about UILabel's behavior that the test was
defining as correct. Instead of assuming that a label's line height will
reset to 0 if its text changes, the test now just checks that setting
the label's line height manually won't be overridden when there are no
longer observers.

Various comments are also added to indicate that there is no guarantee
of what happens when a label stops being observed; it's now OS-version
dependent.

Bug: 978178
Change-Id: Ifa523d1e5cb8eecfbad5ac0a9d56e5d9c55a441d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1683268
Commit-Queue: Mark Cogan <marq@chromium.org>
Reviewed-by: Robbie Gibson <rkgibson@google.com>
Cr-Commit-Position: refs/heads/master@{#674045}
3 files changed