Add flags for WebUI dark mode on Chrome OS

(1) features::kWebUIDarkMode. A base::Feature for this functionality
    already existed, but it was not part of chrome://flags and was
    difficult to enable for Chrome OS users. This CL adds an entry to
    chrome://flags on Chrome OS.
(2) blink::features::kForceWebContentsDarkMode. This flag was present
    in non-Chrome OS builds but was disabled on Chrome OS due to
    https://crbug.com/1011696. This CL enables the flag for Chrome OS
    but does not enable any variants/experiments for this platform.

Change-Id: Ie907255a95c09f3be70bd91f2eb850ca68a08c41
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2486660
Reviewed-by: dpapad <[email protected]>
Commit-Queue: Kyle Horimoto <[email protected]>
Cr-Commit-Position: refs/heads/master@{#819113}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index 0e4576d..3d820a28 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -2615,6 +2615,9 @@
 extern const char kVmCameraMicIndicatorsAndNotificationsName[];
 extern const char kVmCameraMicIndicatorsAndNotificationsDescription[];
 
+extern const char kWebuiDarkModeName[];
+extern const char kWebuiDarkModeDescription[];
+
 extern const char kWifiSyncAndroidName[];
 extern const char kWifiSyncAndroidDescription[];