Add UMA to report Preferences File Corruption

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index e8631e7b587db93a09e4ae84afad03f8bd0eaa0d..474e3041a3e377122b71d6f011369a1ee49a4673 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -13458,6 +13458,37 @@ other types of suffix sets.
   </summary>
 </histogram>
 
+<histogram name="Settings.TrackedPreferenceChanged" enum="TrackedPreference">
+  <summary>
+    The id of a preference whose value has changed outside of Chrome when a
+    profile is loaded.
+  </summary>
+</histogram>
+
+<histogram name="Settings.TrackedPreferenceInitialized"
+    enum="TrackedPreference">
+  <summary>
+    The id of a tracked preference that is initialized when a profile is loaded.
+    This may be the first time the pref is tracked, or the hashed value may have
+    been removed from local state.

[Mark P, 2013/08/20 19:26:25]

This implies that every time something is emitted to the removed histogram,
it'll also be emitted to this histogram.  Is that true?

[bbudge, 2013/08/20 20:43:34]

"removed" here means the hashed value from local state. It's confusing so I
tried rewording it again.

+  </summary>
+</histogram>
+
+<histogram name="Settings.TrackedPreferenceRemoved" enum="TrackedPreference">
+  <summary>
+    The id of a preference that has been removed outside of Chrome when a

[Mark P, 2013/08/20 19:26:25]

please be consistent and either use "tracked preference" or just "preference" in
all these descriptions.  Otherwise someone may wonder if there's a difference.

[bbudge, 2013/08/20 20:43:34]

Done. Note that I've changed this to Settings.TrackedPreferenceCleared.

+    profile is loaded.
+  </summary>
+</histogram>
+
+<histogram name="Settings.TrackedPreferencesChecked">
+  <summary>
+    The number of times all tracked preferences are checked when a profile is

[Mark P, 2013/08/20 19:26:25]

nit: does "all" add anything here?  Is it possible that sometimes only some but
not all tracked preferences are checked?

[bbudge, 2013/08/20 20:43:34]

Actually, this histogram isn't quite right, since it was counting startups. I've
changed it to be like the other histograms and track how many times each pref is
"unchanged". (tip of the hat to Erik Kay)

+    loaded. This is used to normalize the Settings.TrackedPreference* histogram
+    samples.

[Mark P, 2013/08/20 19:26:25]

nit: histogram samples -> histograms

[bbudge, 2013/08/20 20:43:34]

Done.

+  </summary>
+</histogram>
+
 <histogram name="SimpleCache.CheckCRCResult" enum="CheckCRCResult">
   <summary>
     Whether or not the CRC was checked at the moment when the last reference to
@@ -22459,6 +22490,15 @@ other types of suffix sets.
   </int>
 </enum>
 
+<enum name="TrackedPreference" type="int">
+  <int value="0" label="prefs::kShowHomeButton"/>
+  <int value="1" label="prefs::kHomePageIsNewTabPage"/>
+  <int value="2" label="prefs::kHomePage"/>
+  <int value="3" label="prefs::kRestoreOnStartup"/>
+  <int value="4" label="prefs::kURLsToRestoreOnStartup"/>
+  <int value="5" label="extensions::ExtensionPrefs::kExtensionsPref"/>
+</enum>
+
 <enum name="TranslateError" type="int">
   <int value="0" label="No error"/>
   <int value="1" label="Network error"/>