Record stats about kExtensionKnownDisabled with the PrefMetricsService.

chrome/browser/prefs/pref_metrics_service.cc

Index: chrome/browser/prefs/pref_metrics_service.cc
diff --git a/chrome/browser/prefs/pref_metrics_service.cc b/chrome/browser/prefs/pref_metrics_service.cc
index 2ee5e1ae2b49ceb63710c778462df3e242ac14dd..966721c68fbe71bbb1d274ffb0dd79c73fd82580 100644
--- a/chrome/browser/prefs/pref_metrics_service.cc
+++ b/chrome/browser/prefs/pref_metrics_service.cc
@@ -38,6 +38,13 @@ namespace {
 
 const int kSessionStartupPrefValueMax = SessionStartupPref::kPrefValueMax;
 
+// An unregistered preference to fill in indices in kTrackedPrefs below for
+// preferences that aren't defined on every platform. This is fine as the code
+// below (e.g. CheckTrackedPreferences()) skips unregistered preferences and
+// should thus never report any data about that index on the platforms where
+// that preference is unimplemented.
+const char kUnregisteredPreference = "unregistered.preference";

[bbudge, 2013/10/15 18:56:00]

I wonder if an empty string might be a better value. Or you could replace the
'.' by '_' or just 'unregistered'.

[gab, 2013/10/15 19:09:35]

Used "_" -- it's cuter ;)... but more seriously I think that's never likely to
be a real preference name.

+
 // These preferences must be kept in sync with the TrackedPreference enum in
 // tools/metrics/histograms/histograms.xml. To add a new preference, append it
 // to the array and add a corresponding value to the histogram enum.
@@ -55,7 +62,10 @@ const char* kTrackedPrefs[] = {
   prefs::kDefaultSearchProviderName,
 #if !defined(OS_ANDROID)
   prefs::kPinnedTabs,
+#else
+  kUnregisteredPreference,
 #endif
+  prefs::kExtensionKnownDisabled,
 };
 
 static const size_t kSHA256DigestSize = 32;