[iOS][DarkMode] Add metrics for dark mode

Create UserInterfaceStyleRecorder to record the relevant dark mode
metrics. This are "UserInterfaceStyleUsed" and
"UserInterfaceStyleChangedWhileActive".

Bug: 1001192
Change-Id: Iaabb3064316366d9d51e06a782c77dca5156a622
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1787788
Reviewed-by: Brian White <bcwhite@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Robbie Gibson <rkgibson@google.com>
Commit-Queue: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695640}
6 files changed