Improve the comments around content settings histograms.

components/content_settings/core/common/content_settings.cc

Index: components/content_settings/core/common/content_settings.cc
diff --git a/components/content_settings/core/common/content_settings.cc b/components/content_settings/core/common/content_settings.cc
index e7a8dea8fb8ec22b5cf3d3140d5f3b24377e21b2..fae51e139b16a575dce6fed212032df6a7c466e6 100644
--- a/components/content_settings/core/common/content_settings.cc
+++ b/components/content_settings/core/common/content_settings.cc
@@ -17,9 +17,9 @@ ContentSetting IntToContentSetting(int content_setting) {
 }
 
 // WARNING: This array should not be reordered or removed as it is used for
-// histogram values. If a ContentSettingsType value has been removed, the entry
-// must be replaced by a placeholder. It should correspond directly to the
-// ContentType enum in histograms.xml.
+// histogram values. New content setting types must be added to the end.
+// If a ContentSettingsType value has been removed, the entry must be replaced
+// by a placeholder.
 // TODO(raymes): We should use a sparse histogram here on the hash of the
 // content settings type name instead.
 ContentSettingsType kHistogramOrder[] = {
@@ -43,12 +43,8 @@ ContentSettingsType kHistogramOrder[] = {
     CONTENT_SETTINGS_TYPE_MIDI_SYSEX,
     CONTENT_SETTINGS_TYPE_PUSH_MESSAGING,
     CONTENT_SETTINGS_TYPE_SSL_CERT_DECISIONS,
-    CONTENT_SETTINGS_TYPE_DEFAULT,  // METRO_SWITCH_TO_DESKTOP (deprecated).
-#if defined(OS_ANDROID) || defined(OS_CHROMEOS)
+    CONTENT_SETTINGS_TYPE_DEFAULT,  // METRO_SWITCH_TO_DESKTOP (removed).
     CONTENT_SETTINGS_TYPE_PROTECTED_MEDIA_IDENTIFIER,
-#else
-    CONTENT_SETTINGS_TYPE_DEFAULT,  // PROTECTED_MEDIA_IDENTIFIER (mobile only).
-#endif
     CONTENT_SETTINGS_TYPE_APP_BANNER,
     CONTENT_SETTINGS_TYPE_SITE_ENGAGEMENT,
     CONTENT_SETTINGS_TYPE_DURABLE_STORAGE,