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..1002fd968d206f9855d6fd97962adb6328aa0d9e 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -13458,6 +13458,33 @@ 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 preference whose last value is unknown when a profile is loaded.
+  </summary>
+</histogram>
+
+<histogram name="Settings.TrackedPreferenceRemoved" enum="TrackedPreference">
+  <summary>
+    The id of a preference with a last known value that isn't present when a
+    profile is loaded.

[Mark P, 2013/08/20 14:56:18]

How is "not present" different from "unknown"?

Note: in common usage, "removed" is not the same thing as "not present".  i.e.,
histogram name and description do not jibe

[bbudge, 2013/08/20 18:17:50]

I think the name is good, so rewrote the comment. Done.

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

[Mark P, 2013/08/20 14:56:18]

Can we rephrase this to be analogous to the other histograms?  Perhaps
The id of a preference whose value is checked when a profile is loaded.

[bbudge, 2013/08/20 18:17:50]

This enum is a boolean, to give us a normalizing value for the other events. It
records the number of times we check all preferences in
PrefMetricsService::CheckTrackedPreferences().

I reworked the comment to be a little clearer.

+  </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 +22486,15 @@ 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"/>