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This patch adds three UseCounters for measuring dark mode usage:
1. kForcedDarkMode which is set for pages loaded with forced dark mode.
2. kPreferredColorSchemeDark which is set when prefers-color-scheme
 media queries will match dark.
3. kPreferredColorSchemeDarkSetting which is set when the preferred
 color scheme dark setting is set (i.e., the system is in dark mode).
 This can differ from #2 in cases where the system is in dark mode and
 forced dark mode is enabled, resulting in prefers-color-scheme being
 "light" in some cases.

Bug: 1131028
Change-Id: I85bc9db6ef30ce0dfc4f78c0c51d039bb04a100d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2423168Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809477}